Every one of my subfolder PRI files has the line
INCLUDEPATH *= $$PWD
so presumably that results in -I(value of PWD for that folder) so stuff should still be found.
In any case, I repeat my original issue -- in the old Qt Creator, the project was able to find all the files referenced in each .PRI file, WITHOUT the need to prefix the HEADER and SOURCES entries with $$PWD/
That doesn't work any more. So either it was a "flaw" that it used to work, or it is a bug that it doesn't work now.
Can't have it both ways.