I saw something similar today with Open Document Reader. In its case, it’s a manifest failure:
failed to install at.tomtasche.reader_164.apk: Failure [INSTALL_PARSE_FAILED_MANIFEST_MALFORMED: Failed parse during installPackageLI: /data/app/vmdl662567906.tmp/base.apk (at Binary XML file line #64): at.tomtasche.reader.ui.activity.MainActivity: Targeting S+ (version 31 and above) requires that an explicit value for android:exported be defined when intent filters are present]
I believe the issue here is the same as the one andrewc reported and thus not F-Droid specific.
Do note that if you update targetSdk, you are explicitly telling the Android OS you are opting in to new behavioral changes, so you should be extra careful, read the release notes (especially the part “Behavioral changes”) and test thoroughly before you release a new version with updated targetSdk.