FaberCastell 1.1.28 vs .37...
I see version 1.1.36 on Fdroid, which makes sense since 1.1.37 is less than 24 hours old and will likely not be on Fdroid for at least a few days.
Did Graphene reject the install from fdroid because it was too old?
No idea. I'm also not sure that Graphene would do that. As far as I'm aware, the only reason that the OS would reject an install is if the version was older than an existing install. Taking into account that Fdroid is apparently showing you older versions of Obtainium, its possible this might be more of an Fdroid problem. If thats the case, I won't be able to help as I don't really use Fdroid.