The circuit with the fault is on an extended ground. It was in a wire nut but corroded and loose looking. I took that apart, shined all 3 wires in the bundle, and put it back together with a wago.
No improvement.
Discussion
Is it a partially open ground or a partial short to hot? I shut off the breaker and tested continuity in an affected outlet, no continuity between any leads.
In that case, I could just run a new ground into the circuit and call it a day. I think I want to be sure I found the issue though. That might leave some things unfixed.