apparently the brokenness of the tags and content warning fields in the post composer are related to widely-known issues with input events on android software keyboards being a complete nightmare that doesn't match any other platform on earth. this thread tipped me off that most of the problem is fixed in android 12L aka android 12v2 aka "API level 32", which I was able to verify with a device emulator; the fields work as expected with gboard in firefox.
unfortunately, I have no idea if or when you will ever get android 12L on your device.
edit: as @shork pointed out down in the comments, it appears to also be broken on deployed android 12L devices with newer, production builds, so maybe the behavior was changed accidentally during development and they changed it back before ship? I love computing.
