Spoke too soon. It's happening again.

Update: updating the phone to the latest Android 16 seems to have improved reliability. Will update if it starts happening again, leaving this here in case it helps someone else with a similar issue.

Seemingly the only way to get Helicon Remote to detect the camera is to connect the cable, then open and close Helicon several times until I see that it opened in landscape mode with all the buttons greyed out. If it opens in portrait mode, it has not and will not connect, and I need to force quit ...