MetropleX
None of the links you provided are relevant to the specific issue experienced in this thread. Links 1 & 2 don't describe anything comparable. 3 is specifically "Scan QR code from EE", likely network locked. 4 is unrelated.
One of the first things people should do before instantly thinking we have a process breaking bug
Nobody here "instantly" assumed GrapheneOS had a process breaking bug.
Multiple people have flashed stock, tested, ordered new phones, tested, and narrowed it down to the process of flashing GrapheneOS. I have confirmed this with two Pixels, and so has aceblack and a couple of others.
So it's quite reasonable that we expect the install of GrapheneOS the be the cause.
Something people could try is grabbing the QR data and seeing if they can add it manually which appeared to work for one user in the previous links.
This has already been tried in this thread and does not work. The eUICC appears to be stuck in a read only state.