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

Izzy is a repo for fdroid, on Izzy they can still host your app even if it isn't RB. Also, they just scan places like github releases and post your apps automatically for you, while fdroid only includes it if you submit it first.