Phone rings on incoming call, but screen doesn't show caller or buttons

Expected Behavior

When I receive an incoming call, the phone should show the caller name, and the accept/decline call buttons.

Current Behavior

Intermittently, the phone audibly rings, but doesn't show the accept/decline call buttons.

Making an outgoing call puts the current incoming call on hold.

Steps to Reproduce

Not sure, as it's intermittent - keep calling, perhaps? There doesn't seem to be any pattern (e.g. time of day, caller) that I can see.

/device river /version 20-20240120-NIGHTLY-river /date 2024-01-05 /kernel 4.9.333-perf+ /baseband M632_43.192.01.165R /mods Google Apps

I have read the directions.