That's unfortunate. I did manage to narrow down the problem quite a bit more though which might also help with reproducing it. The problem actually doesn't occur with the first reboot, as I first suspected, but rather it occurs after you turn off the screen/put the phone to sleep.
So immediately after you boot up the phone the camera still records video fine (sometimes the app crashes after reboot though and cannot be opened, another reboot fixes it). But after you put the phone to sleep by pressing the power button (probably also if it goes to sleep automatically) the discrepancy starts showing up. So if you put your phone to sleep for five minutes and record a two second video, the resulting video will be 5:02 with the sound at the beginning and video at the end. If you put the phone to sleep again, the video length of the next video increases by about the same amount.
So the video length = actual video length + (time phone has been asleep since last reboot - 10 seconds or so). If you only put the phone to sleep for a few seconds, the discrepancy doesn't start yet, it has to be asleep for ~10 seconds or more.
So if anyone has the same device, they can try reproducing it by booting up the phone, taking a 5 second video, putting the phone to sleep for a few mins and then taking another 5 second video. The second video should then be of excess length.
So my guess would be that either the audio recording and video recording functionalities use different variables for timekeeping in the first place (time active vs time turned on) or that putting the phone to sleep kills some sort of time keeping functionality (maybe to conserve battery?) that the audio recording relies on while leaving the video equivalent untouched or vice versa.
