[BUG] Possible bug when unlocking 2 achievements at the same time

One of my betatesters got an achievement partly unlocked at 10%.
The problem is that this achievement only ever unlocks to 100%. I have checked and doubled and triple checked, and everything seems right on my part.

The only thing I can think of is that it got unlocked at the same time as another achievement, which seems to be the case, since he also got another achievement at 10%, and that is a valid one, and in my game it is possible to get those to at the exactly same time.

Unlocking an achievement is done by sending a very precise key name to Apple. I do it one at a time in a specific routine.
So this seems like a very strange problem.