i wonder if drive format and/or file structure has something to do with this.

maybe those who did not have an audio resume issue can share how their drives are formatted, etc

I tried modifying my previously working USB stick and it still doesn't work.  (i.e. I added new files to the USB stick hoping that the unit would re-index it and possibly fix the resume functionality, but it still doesn't work)

Pioneer is still telling me it works on their end.

Are you fat32 or NTFS?

my sdcard and ssd are both fat32. From what i can tell by posts, the 4000nex, does not have the problem, but the 8000nex does. 

The 4000NEX most-certainly does have the problem!  That's what I own.

It cleared itself -- but again, it was after roughly 10 hours in the car, with a few breaks for fuel and the can, music playing off the SSD the entire time.

In my opinion, if it "cleared itself" it is not a problem. I already stated that it took some on/off cycles to start resuming. I have read that on some 8000 nex's, they never got to the point of resuming. That would be a problem. Taking a little more time than previous, is more of a nuisance than a problem.

It's not "a little more time", it was a literal all-day drive in the car on the back of a previous day in which I was in the vehicle for a couple of hours, and during which it did not clear.

This implies (rather strongly!) that if I remove the drive to add media to it then in "ordinary use" it will effectively never clear, as it's not all that common that I take all day, 700+ mile road trips -- and yet it was not until I did take one that the problem went away.