10 s and 60 s intervals with 4 kHz/256 smpl produced stable timestamps.
Hmm.. I think I found out what is happening. There has to be wrong type of variable somewhere. App source has logging interval options in milliseconds, so one minute is 60000. Unsigned 16 bit integer goes up to 65535 and one minute interval just fits in. 10 minutes goes over that (actually 9 times) and then leaves 10176 milliseconds as logging interval.
Now we need @admin or anyone who knows anything about coding to help. I have now idea how to track where this happens and if it is in the firmware, only James could solve this..