i've been working with a fireface UC for two weeks, and i'm very impressed so far, specially with totalmix, but yesterday i had a very strange problem and want to know what was the cause to prevent it from happening again.

i was recording an acoustic guitar with two condenser mics. the recorded track was clearly out of time, it was recorded late. it wasn't a playback latency problem as the wave graphic inside the clip was indeed off the grid. the sound was monitored through totalmix without latency. the buffer was at 1024.
i rebooted the pc and the problem was still there. changing the the buffer to 512 solved the issue.

today i opened the same project in order to make some tests, set the buffer to 1024, but i couldn't reproduce the issue
my daw is protools 12.5 (using low latency monitoring), my OS is win 7.
i never had latency in audio recording before even with low quality interfaces. so i don't know what could be the cause to the problem.