I tested this myself now. Android 12 and Diarium v2.9.29 and Dropbox sync.
I wrote some text and minimized the app. Opened it back up right and the data was synced.
So I am not able to reproduce this problem.
The app also does not sync when you open up the app again?
On the top right it shows the exclamation mark in the sync icon after you open up the app again?