Solved, thanks to your feedback, which lead me in the right direction!
In Xiaomi MIUI 12.5 (android 11): Settings > display > more dark mode options > individual apps:
Turn off dark mode for Obsidian. Inside Obsidian settings: turn on dark theme again, as it will have flipped to light theme now.
Now it works.
So there is indeed an OS dark mode theme per app instead of the app based theme. I haven’t had issues before, I’m not sure exactly how this is supposed to work.