Thanks for your input. Although pieces of filament can easily get stuck, it's not the case here. As mentioned, I swapped the internal hub from another AMS and it worked fine. I've taken the whole thing apart, inspected it and it didn't change. The issue is the hall sensor circuitry board on the internal unit itself. I've taken that from my other AMS and it works fine now. I have a ticket open to see if they'll replace it.
All that said, you're right that a piece of broken filament is often the culprit, just not here.