Yes, annoying vegans who are very pushy about vegans exists. However, It seems like your bias stems from your intolerance towards any mentioning of veganism.
A vegan saying ‘I am vegan’ would be annoying to you, as if existing as a vegan is an offense. This is what you sound like when you say veganism is a yellow flag.
Personally, I find that there are far more meat eaters out there who are much more vocal and annoying about hating vegans than there actual annoying vegans. I like eating meat, but I don’t find that I need to be defensive about it around vegans.