Interestingly, I have subsequently seen a similar pattern of behaviour associated with an uncaught error (i.e. the ‘_lastrun.py’ file fails to update and any changes made to the program do not show up when running the program) in v2020.1.3, this time caused by a different event. In this case, the event was not a change made to the program, but the act of moving the project folder into the Windows 10 recycle bin and then retrieving it. The solution was to rename the Builder program, either using ‘Save As’, or just highlighting and changing the name of the file in Windows Explorer.
The Builder program had somehow become self-contained, so that it ran even when all associated Excel conditions files and image files were deleted, and even when the ‘_lastrun.py’ file was deleted. Renaming the program freed it from its hermetic bubble. Changing its name back to the old name put it back into the bubble. All perfectly repeatable.
So, if anybody ever notices that changes they have made to their Builder program are not shown when it runs and there is no error message, it may be worth checking whether the ‘_lastrun.py’ file is being updated. If it is not being updated, then renaming the program may solve the problem.