It's really not. That's why I keep putting "rinse" in quotation marks. In your case, the tank supplies water to the high pressure pump for every function. I was trying to determine if the water with rinse selected was coming from a different source from a solenoid, but it looks like it's not. Soap, wax, and rinse all pull from that tank.
I assume you never ran this test I described in a previous post. IMO this is the most likely cause of your problem.
A low pressure chemical could be backing up into the tank. I have seen this happen many times, and it requires a bad pump inlet check valve AND a bad pump. (Addendum: you don't even have a check valve on the pump inlet) The easiest way to check for this (assuming the spot free check valves are at the boom) is to run each bay on a low-pressure function (tire cleaner or presoak) for 30 seconds, don't need the trigger pulled, then run it on spot free with the trigger until it runs clear plus a good 15 seconds or so, then run it on rinse. Nothing but water should come from the gun. If you get any significant amount of the low pressure tire cleaner or presoak, the pump is the problem. Check all the bays this way.