Fdroid requires app to be RB (reproducible build) which is good cuz it mean the apk developer release should be identical to a build you make from source code.
My issue was that updating or adding a library to my app could break RB which can be a simple fix, but would require either waiting for pull request to be merged or forking yourself.
It just becomes a game of Guess Who to find who is at fault for RB issues. Maybe it's just a flutter thing and isn't an issue with kotlin apps, idk.
IzzyOnDroid is a great fallback