This happened to me today in fact, I've started to email with Zaxcom support. What I noticed is that there was an exclamation point next to the hours meter (timing remaining), before I manually stopped the recording.
I'm going to test tomorrow if the behavior is the same if I eject the card - that way I'll know if its perhaps the card that is faulty, the one that was supplied with the unit - I've not had this happen with sandisk cards on my other transmitters.
Oh, and be sure to contact Zaxcom support, I'm going to see if they want the files off my card to see if there is anything that they can figure out from looking at the files themselves.