This isn't power gaming. It's just smart play. Automatic success is better than leaving it to the dice to decide, which is why something like portent is a good character option.
Literally every element of power-gaming can also be described as "smart play" (or "smart character building" or the like). So that's meaningless.
Getting automatic success without something like portent requires engaging with the environment or the NPC in a way that removes uncertainty as to the outcome and/or the meaningful consequence for failure which by any reasonable standard is "good roleplaying."
Disagree. It's only good roleplaying if it's plausible for the overall character you have. If you go out of character, or just don't even have a character, no matter how much you "engage with the environment or NPC", that's not "good" roleplaying, that successfully manipulating the DM.
My personal experience is that the kind of people who want stats to mean absolutely nothing tend to have characters who have absolutely no discernable personality, motivation, goals, or the like - they may have a backstory, even a detailed one, but in-game they just say/do whatever they think is going to be most successful, even if it makes zero sense for the character they seem to have.
If you never don't say/do something because your character wouldn't do that, or wouldn't know that, you're definitely this interesting species of power-gamer, potentially even a munchkin if it extends into making the game boring or annoying for others, no matter how effective it is (which is largely up to how well you can make the DM do what you want).
What sort of bad behavior have you seen?
Largely two things:
1) People not role-playing their character at all, or just stopping RPing their actual character at times,
because it might hold them back, in groups where most people do RP, even if sometimes that means their character doesn't make the most optimal possible decision (which, in my experience, and YMMV, is most groups). There's always going to be a little bit of this, and that's fine, but I'm talking about when it's extreme/constant. As I noted before, I've seen characters with detailed backstories, and in a couple of cases, even written personalities, but then the guy just doesn't RP it at all, and ignores his stats too (and alignment, of course), and just tries to manipulate the DM into letting him not roll (which again, I can sympathize with, but only up to a point), and basically plays an emotionless (or fake-emotion) psychopath who will say/do anything to further the party's goal. They're not necessarily causing problems for the other players directly (though this is prone to steal spotlights and invalidate skills that other players too), and may well be on-goal, unlike say, a munchkin who tries to turn everything into combat, but over a few sessions it can really stand out that everyone is RPing, except the dude who is just "playing to win" and has basically forgotten his character even theoretically has a personality.
And if you point out that this is going, the person tends to get very defensive, no matter how politely it's done, because you're calling them on not RPing, which people tend to see as impugning their honour.
2) People bringing in OOC information of all kinds, sometimes basically entire scientific treatises, with absolutely no attempt to justify it via skills/stats/etc. possessed by the actual PC (sometimes not even their background). This is more common in modern games than D&D, but in D&D I've seen it a bit. Again, some limited amount of this is to be expected, but you run into situations where, for example, a character with zero science background and low mind-related stats is explaining in detail how to make gunpowder - in a fantasy setting, especially a wacky one where the elements might not even be real things, you may be able to futz with that by having them be wrong - not so in a non-fantasy setting, if they have the correct information (in my experience, players often have the advantage of the DM here).
It often feels particularly dubious because in practical terms it's generally only possible to do this with stuff covered by INT/CHA, and 5E highlights this by these being the key dumpstats for 5E
Separate from all that my personal feeling is that if your stats are high or low, you should explain it in some way, and RP it in some way, not just
flatly ignore it except mechanically. It doesn't have to be some pre-determined, DM-specific way - if you have a creative explanation for 5 INT that isn't "I R DUMB", like the alien-mindset lizardman mentioned earlier, that's awesome, and probably makes for a memorable game. But when you pick 8 CHA and just act like your character is charismatic, and RP on that basis, without the slightest hint as to why they have 8 CHA (it could be a lack of self-worth, or just a lack of presence despite being perfectly nice or whatever), I feel like at best that's lazy or weak RPing, and at worst, it's trying to get around a dumpstat in a dodgy way.
This is one of the issues I have with 5E, note, and indeed D&D in general. I feel like there are various approaches other games use that work better, and D&D could work a lot better with a few changes (mostly to make the d20 variance less of an issue, but also to boost the value to investing in a stat, outside of to hit/damage/saves), because currently there's a huge premium on just avoiding rolling if you possibly can.
