The problem goes away if I connect the repeater router to the main wifi instead of the guest wifi. Thanks for the tip!
My main router is in AiMesh mode. That seems to be the difference in our setups. I tried again today and can still reproduce the original problem.
After upgrading to 386.3 from 386.2_6, I noticed my guest router, which connects to my guest wifi in repeater mode, no longer gets assigned an IP from the main network and instead runs as 192.168.1.1.
Main setup: 3x AC68U in AiMesh with a guest network in 5G band.
Guest setup: 1x AC68U in...