Rubokopter
I'll try to keep this short.
According to GrapheneOS's lead dev, it's because F-Droid updates itself and the apk on their website is not the most up to date version.
The easiest way to get around this would be to uninstall F-Droid from all profiles, then install F-Droid in owner, then push F-Droid to profiles by doing Settings > System > Multiple users > *the profile you want to install to* > install available apps > *turn on F-Droid*