The relationship of many American leftists to patriotism reminds me a lot of those guys who grow up in oppressively evangelical communities and decide to express their disillusionment with it by becoming black metal satanists - a oppositional mirror of Christianity - instead of becoming atheists.
Like the correct response to the realization that nationalism is a false consciousness used to suppress class conflict is to focus on raising class consciousness not to root against the nation everyone around you is rooting for.
Flag burning anti-patriotism is stupid in the exact same way that jingoistic chauvinism is stupid but it's also stupid in an additional self-defeating way because it sets your goals up in opposition to a bunch of symbols and institutions that normies have been trained their whole life to believe in.
Nations are lines someone drew on maps and flags are pieces of cloth. Made up! Don't get mad at something that's made up especially if it's also alienating people that you need to reach for no good reason!
Ya I feel sometimes like this stuff is the foreign policy equivalent of those straight edge punks who'd tattoo X's on their hands or whatever bsky.app/profile/thre...
I suspect the appeal in part is that it's easy and clear to be oppositional. Power building is complicated, sometimes involves compromises, sometimes fails abjectly, and always involves a shit-ton of work.
This is why I content myself with hating Phish instead of starting a better band.