An update on this:
It was a rogue Theme Component: 👨👨👧👦 Groups Widget (Layouts Plugin)
@keegan, would you be willing to take a look at it? Where would you like me to post more details? The topic here in Meta or in thepavilion.io?
Update
The problem was I had the widget active for trust_level_0 users, which I now know includes Staged Users. Bumping it up to trust_level_1 should have fixed my problem.