Got the same problem with Nest Mini Stereo and Nest Audi Stereo
configuration. No root cause identified but it think it's somehow
related with the power supply. My speaker suffers dirty current that
comes from big mashines. Disconnect power from both...
I got an similar configuration and use 7 speaker in a room with a
tripple stereo configuration. With the 7th, a Nest Hub 2'nd generation,
i connected a bluetooth transmitter as a bluetooth speaker and this way
i am able to operate an active 700w JBL ...