Good to see you're on the path. I happen to be BJCP Certified, took my first exam in 2006 or 2007.
I can honestly say that homebrews do NOT all taste like "homebrews" to me. Of course some are better than others, and the extract beers in particular tend to turn me off IF I am able to taste "the twang", which is often but NOT always the case. Meanwhile, some examples are very very good, some even world-class or better than you can buy commercially. I have judged many 40-point beers in my time as a judge, and maybe even more often than most would expect. If a beer is really good, I am not at all afraid to give it the score that it truly deserves, and I'll defend it if necessary. The opposite is true as well. I'm not afraid to give a score of low teens if it sucks. However the majority do in fact fall someplace around a score of 28-33 which is about what most judges would expect I think for an average quality beer. Maybe even higher, maybe 34. I've found that at least in the USA, average quality of homebrews and commercial beers alike have improved by several points since about 10 years ago.
Phenols? Gosh... that's got to be either a Belgian thing (you're judging in Belgium -- yes? no?), or a personal sensitivity thing. I wouldn't be terribly surprised if brewers in Belgium picked up a certain "house" phenolic just by being where you guys are. But even more likely: Some people are more or less sensitive to certain things than others.
A quick story: I know a National judge who can't taste diacetyl at all. I'm not hyper sensitive to it either, however I've "defeated" him in battle before where he loved a beer, scored it in the 40s, but then when I tasted it I told him it's alright but has some diacetyl so I'd only score it a 30, then when another Certified judge broke the tie, the third judge agreed with ME while the other guy was forced to confess that he knows he can't taste diacetyl so as much as he personally disagreed, he had to defer to us. I'm also a little more sensitive than some people to phenolics I think, and maybe DMS but not as much.
Another example: I absolutely hate that phenolic Carmex lip gloss flavor. This actually caused some additional hate and discontent when I received the results from my last BJCP tasting exam, where a Belgian dubbel was praised by the proctors for having low phenols, but to me tasted like lip gloss and turned me off, resulting in a difference in proctor vs. my score on the tasting of like 15 points or something, so of course they marked me down for that. I still think my personal opinions are more important than others' sometimes, and for that I guess I'm paying the price, as I'll have to take the exam again if I'm ever to become National. Oh well, someday maybe I'll learn. For now, sorry no, I've not learned yet.
You may be onto something very real. Maybe all the beers you tasted really do have something off. Or maybe they don't. I wish I could taste them right there alongside with you. In fact, don't be afraid to table-talk with higher ranking judges after you each take your notes. You might learn something but also you might even TEACH THEM a thing or two that they might otherwise miss. Personally I don't respect a Recognized judge's perceptions any less than a National or even a Master. We can all pick up on different things, and sometimes a new guy's perceptions can be refreshing and educational if they can describe well enough what they are tasting. "Sour Cheese Nips" has been part of my vocabulary more than once, and occasionally someone else will say "Yes! Exactly! You hit the nail on the head!" Trust your perceptions, because hey, they may very well be right.