Ducter lineage
It's my understanding that they're behind on updates and their patch level isn't exactly 100% accurate, so it's possible that a new bug was introduced at some point by upstream AOSP that just isn't in LineageOS. The reason I think it's unlikely to be because of a change GrapheneOS made is because GrapheneOS, to my knowledge, hasn't changed any code that would affect this sort of thing.
I am sure people want this fixed, but it's not really possible for the developers to troubleshoot and fix a problem when they don't know how to reproduce it. I'd help if I could, but I seem to always get notified of all incoming calls, even if I actually end up ignoring all of them.