After doing several hell cycles this weekend I checked my skulls to make sure my toon tracking spreadsheet was up to date, and I noticed that several of my toons have the Avernus tag on their skull, when I didn't run a Tia after completing Nessus.
Example toon is Nymeria Sand on account Tjsail34. We completed a Ness shortly after midnight ET last night, which was Nym's third demi. I stopped playing last night after doing that, have done no runs today, but my skull says that I have completed Avernus. I got the third demi as I expected though, so it seems to just be an issue wiping the Avernus tag after completing Nessus.
I can verify this bug. I had a 0x toon that I just took to Nessus today. I checked the skull after and it said I needed all tags for the next hell cycle. However, I just did a stygia and now the skull says I have Avernus and Stygia. I did not do a Tia between Ness and Stygia today.