Maybe this is a know issue with TPU, and I am just discovering something that is widely known, but I though I would record it for posterity if it can be helpful for someone else.
TLD:DR if your TPU stops extruding and then won’t extrude again, it could be because it is getting elongated and thinned. You have to trim of the elongated part of the filament to get it to feed again.
Twice I had TPU 95 HF just stop printing mid-print and then couldn’t get it extrude after that. I ended up replacing extruder gears and nozzle etc. Thinking that those parts were worn out…Maybe, but also discovered that the TPU was getting elongated (I assume by the tension from the spool to the extruder drive) during the failed prints. then when I fed it back into the extruders, it was too thin to be engaged by the extruder drive gear and would just sit in the printer head melting.
After cutting off about 12 inches of TPU I found the it would print again.
Also I think 12mm^3/s might be pushing the volume just a bit too much for my setup which is why I was getting the elongation in the first place.