LMS won't talk to piCorePlayer on same rPi

I am running piCorePLayer with included LMS on a pi4, with USB out to a USB-DAC->amp. LMS has the Airplay plugin installed. Music files are on a connected USB SSD.

Sometimes I control all this with SqueezePad on a iPad. This works, both controlling play on rPi pCP renderer or on networked airplay devices as intended.

Trouble arises when using the Logitech Media Server HTML GUI from a MacBook on same network. Here's what happens:
> Select the pCP player from the dropdown on the GUI's upper right
> Select Play on an audio file from the LMS media browser on the left
> Selected file info won't show up on the player queue panel, and won't play through the rPi local renderer.
> After 3-4 seconds, player dropdown switches by itself to the MacBook
> Selected file info shows up in player queue panel, and audio stream to MacBook commences (where I don't want it)

Looking at all the various settings, on Squeezelite, LMS and its plugins, I can't seem to find the problem. Control of play using LMS web GUI on MacBook was working fine a while back, and don't know what I may have done to break it.

Suggestions?