This sample video file is created with #timestamp# Text filter on #000000 background with some lighter edges beforehand. No filters were applied after I opened this exported file.
This happens in 18.09.16 but not in 18.08.14 and 18.03.06.
The added or subtracted frames were typically 4 frames but on few occasions 3 frames.
If snapping is disabled, the added or subtracted frames get reduced to 1.