The Pale Ale that got terrible marks had numerous remarks around a phenolic off flavor (smoky/plastic). Two score sheets said the same thing (although I've been told judges score together so perhaps they influence each other).
Peoples' perceptions differ, and some times certain hop blends can give the illusion of phenolic character. And, as you said, judges' perceptions can be flaky, even for higher level judges. (Although a National or better judge will be accurate more often than not, and should be able to explain exactly what perceptions they detect, and give correct suggestions for how they think those faults can be fixed.)
Additionally, many competitions use hard plastic cups which are lined with mold release compound which has a distinctive plastic smell. Unless cups used at the judging table are aired out prior to use that smell can linger and fool inexperienced judges into thinking there's a problem with the beer - particularly in the aroma.
So, it might be nothing at all, and just bad luck with judges.
But, in addition to the good suggestions provided so far:
1) How much caramelization/melanoidin development (i.e., darkening, toasty or "darker" malt character) are you getting during mash/boil? Do you suffer from scorching on the bottom of your boil kettle? Does your boil kettle have a layer of scorched/carbonized wort on the bottom?
Scorching, or even just extreme levels of caramelization, can produce smoky phenolic notes.
The fixes are to turn down the heat on your heating element during mash/boil, put a flame tamer under your boil kettle/mash tun, and/or go to mashing/boiling vessels with better conductivity and thicker bottoms.
If it's present, do what you can to remove as much scorched material from the bottom of your brew kettle.
Smoky/plasticy phenols are unlikely to a result of polyphenols/tannins extracted from the mash by mashing out at too high a temperature, by somehow carrying grain particles into the boil, or by adding too much hop volume. Polyphenols will give a harsh, bitter flavor, which can sometimes seem "oaky," or "barrel-like," as well as distinctive drying, astringent mouthfeel character which can be mistaken for mouth-coating hop resins, acidity, or high mineral levels. pH above 5.8 in the mash, or prolonged temperatures above about 168 *F when mashing out or boiling, favor polyphenol/tannin development.
2) What's your cleaning/sterilization procedure like? In addition to the perils of using too much chlorine-based cleaner/sanitizer, wild yeast - which can throw plastic/smoky notes - can hide in cracks or crevices in your system, including areas where dirt isn't removed. You can't sanitize dirty equipment.
Take a long, hard look at all your cold side equipment, inspecting it carefully for cracks, pits or scratches. Are there bits of equipment that you're not fully taking apart where bugs can hide? Perhaps replace soft plastic cold side equipment like hoses or brew buckets. Make sure that you're properly cleaning your equipment - both to eliminate dirt and to prevent pitting or scratching.
3) What's your packaging procedure like? Often times, brewers will really sweat cleaning, sanitation and rinsing for their cold side equipment, but not for their bottles or kegs. And, its really easy for wild yeast or other bugs to take hold in dirty kegs or bottles.
All the suggestions that apply to fermentation equipment apply to packaging equipment and containers.
Additionally, if you're using iodine-based sanitizers, make sure you're not soaking bottles/kegs in a too concentrated solution, and make sure that containers rinsed "no rinse" sanitizers are fully dry before you start packaging. Otherwise, you can get distinctive aromas/flavors, which can sometimes be mistaken for smoky/plasticy, in the finished beer.
4) What's your hop blend? Does it include high levels of hop strains which commonly get described as being "spicy" or "woodsy". Its just possible that to certain palates that hop blend might seem smoky.
5) What's your "yeast management" procedure?
If your yeast is contaminated with wild yeast, you can get smoky/plastic notes. Excessive yeast stress due to high temperatures, poor wort oxygenation, and/or underpitching can cause some strains of brewer's yeast to throw phenolic notes (typically clove/spice, but occasionally smoky or plasticy).
Are you pitching a sufficiently large, and sufficiently healthy starter? Are you using the same good wort production and cleaning/sanitization techniques you'd use in the brewhouse for the wort and equipment you use to make your yeast starter? Are you oxygenating your wort to achieve the proper levels of dissolved O2 for the wort's OG? Are you fermenting at the proper temperature range for your yeast strain?
If you're reculturing your yeast, are you using proper sterilization/exclusion procedures to get pure samples from batch to batch?
6) What's your water like, and what's your water treatment plan?
I've left this for last, since it's the least likely option since you said you're dechlorinating your water.
Unless you're living in a part of the world with water that needs to be extensively modified in order to make it suitable for brewing, and you have the knowledge, means and equipment to "built your brewing water from scratch," I'd recommend against using pure RO or distilled water. Your mash and your yeast need certain mineral and metal ions to be present at certain ranges in the water for optimum performance.
For most parts of the country, dechlorinated tap water is suitable for brewing some or all styles of beer, although for the large swaths of the country where there are high levels of temporary hardness, you might need to decarbonate your water prior to brewing the lighter styles. For the areas of the country where there are high levels of permanent hardness, you probably want to amend your dechlorinated tap water with RO/distilled water to get the mineral levels and pH right.
A final possibility is are you somehow getting chlorinated water into your beer? For example, if you've got an immersion chiller, a leaky faucet attachment or hose on the inflow side can allow chlorinated water to get into the wort. Check for leaks!
Are you rinsing your cold-side/packaging equipment with tap water and leaving significant amounts of water in the containers before you run wort/raw beer into them?