Here's a snippet from a process builder formula that was working in production (or at least not giving syntax errors when saved) until recently.
This is part of a formula in a process that runs when a contact is saved or edited. This is attempting to reference data on a related record. I have to assume that it was working in the past (or at least not causing errors in the process.
Apex test code that worked a month ago started failing this week. That test code has not changed. The reason for the failure was the process that contained this condition. The error for the test code points to the process, but the error message in the log shows the following (summarized):
"Invoicing_Contact__c is unsupported in ISCHANGED and PRIORVALUE."
The documentation on PRIORVALUE does not mention that lookup fields can't be used in PRIORVALUE. The process can be saved and no error message is given at that time. Is this something that has changed with the Spring 21 release? The timing makes this a possibility, but I see nothing in the release notes that appear relevant.