OK,
So, the normally Open Dema solenoid is likely not closing fully (like I've never seen THAT happen) when the system switches to winter foam.
This would allow "contamination" of the winter foam diluting the freeze protection. Meaning you're likely getting a weaker than desired protection package.
Combine that with the 8' of unheated trough and...
The reason I asked about the pump cycling when the car wash is idle? The liquid solenoid for that bay may have a very tiny leak. This, will cause the
FloJet to send an occasional pulse of juice into the foam brush supply line for that bay. And with a diluted winter mixture, the unheated trough, and subfreezing temps, the next morning that one brush is frozen...all the way up to the heated portion of the trough.