Summary; BTT is working again but not sure why (or why it stopped working for 24 hrs). I am documenting my summary in case it helps anyone else.
Basic problem for me was all of a sudden BTT didn't work right. It would load/run, but none of my icons would appear, just blank on the touchbar. I did toggle the show hidden option to no success. I restarted BTT and the MBP machine multiple times to no success.
I then uninstalled BTT by moving the app to the Trash bin, restarted macOS. Then I reinstalled BTT using the link provided (it was the same version I had previously installed/running), then re-added the Privacy allowance settings as indicated but still nothing worked (no custom touchbar icons loaded) even though BTT was running.
Once I did see briefly my BTT icons load, but then they disappeared after a few seconds. I could repeat this if I restarted BTT, but ultimately after a few seconds the icons in my touchbar disappeared... very strange.
I closed my MBP for the day while at work today, now tonight upon logging in I toggled the unhide menu option, then everything now works again as normal.
I am not sure what in all of this "fixed" the problem, but mainly happy things are working again.