I’m not sure if your post is about you, but I think it is so I’m going to reply as such, but if it’s not I apologise.

There’s no single way to define anyone, autistic or not, but if you’re exhibiting fawning behaviors and shy away from confrontation it can be perceived as lacking conviction, confidence, a firm sense of self. This isn’t exclusive to autistic people.

When I think of a “Nice Guy” I think of someone that is performatively nice to women, not for the sake of being nice, but to achieve a more underhanded goal (i.e. sleeping with them). One way this can happen is by fawning over them and love bombing. The niceness comes with conditions and so it’s not authentic.

All that aside, I think you have a warped idea of what a “Nice Guy” is. People don’t go so far as to think about “hidden strength, quiet intensity, or restrained anger”, it’s probably more surface level. If they feel uneasy it’s more likely because they can’t read your mood or emotions, and are concerned about you suddenly going into a rage with no warning signs.