Once it passes inspection, the F-Droid build service compiles and packages the app to make it ready for distribution. The package is then signed either with F-Droid’s cryptographic key, or, if the build is reproducible, enables distribution using the original developer’s private key. In this way, users can trust that any app distributed through F-Droid is the one that was built from the specified source code and has not been tampered with.
https://f-droid.org/en/2025/09/29/google-developer-registration-decree.html
TBH I would actually expect GrapheneOS not to disable these checks. GrapheneOS devs pride themselves to have the best implementation of the official Android security model, and enforcing signature checks is likely part of that…
They might add additional certificates I guess, to allow their own apps, and maybe a selected few others.