So many many thanks for your broad and complete answer. You give me new ideas to try.
The way that I had updated my BBB was running: sudo apt-get update and sudo apt-get upgrade. And the PWM subdirectories disappeared.
I installed a more recent version of Debian buster from here: https://forum.beagleboard.org/t/debian-10-x-buster-monthly-snapshots/31203 and update the kernel to the same kernel that you: 4.19.94-ti-r71, but I experienced the same problem.
Now I am going to have to restore my BBB with the old version of Debian from https://beagleboard.org/latest-images while I can find the solution.