Further to my last message...
I've watched the prices whilst MFP is in the "£-1.00" state and they don't seem to update, but the prices then carry on once the balance is correct again. Also, I've noticed that occasionally my scheduled search for new markets is missed, so I wonder if a scheduled search happens to coincide with the "£-1.00" state then nothing comes back.
I added a trigger earlier that runs every few seconds that will record the current time if the balance falls to £0 or below in both of my instances. I've put them together in the attached spreadsheet so you can compare the different timings and see how often this occurs and for how long.
I'm on the latest version of MFP 8.8.1.1