when run / terminate happens, build folder is duplicated recursively

I ran my project in 8.0.2 for years; all was well. Then, after running the program, the build folder expanded vastly. Before running you find

<project folder>/build/classes

and in "classes" you see what you expect: the packages for the project. After running, you have instead

<project folder>/build/classes/build/classes/build/classes/   (continue for multiple iterations)

creating multiple copies of all classes.

This at one time created a build folder with more than 120,000 items (the normal number for my project is 961 items.)

I have installed netbeans 12, and the same project has the same problem - build folder duplication. The symptom that I first noticed was background scanning took 8 hours (although it DID eventually finish.)

Does anyone know how to change the IDE's behavior? If, in order to answer,  you need logs of some kind, I've never done that, so be sure to state what I need to do in a complete way.

Somehow, when saving information from one run as a resource, my instance of the IDE thinks it's supposed to perform this duplication.