I’ve seen this occasionally in my flights if the takeoff altitude was not determined correctly, or if I forgot to start the logger before the tow started. I dont recall if it is OLC or the logger which adjusting your takeoff altitude to airport altitude, but if there is mismatch there, the rest of the altitude trace will be wrong.
The solution is to edit the flight in OLC and manually adjust the start to your release time, which seem to fix this issue.

Ramy