I'm not sure if being on graphene and then getting an update maybe makes the missed calls random or what but for me, clean install and it happened every time. Couldn't get the phone to ring at all. If it was random, happening seldomly maybe I wouldn't have noticed right away but as a small business owner every call is important.
I know a few weeks ago lineage had to do a hard pause for several weeks while rebasing so perhaps it is an upstream issue that they fixed or something... Idk. Hopefully the graphene team can figure it out.