This strange behavior is presumably caused by joining/splitting a stereo track normally never changing the height of the resulting tracks, but collapsed stereo tracks being half the height of 2 regular collapsed tracks.

This behavior would probably be more problematic when there are more than 2 channels, since a collapsed 4-channel track would expand to more space than a 2-channel track.  However, that can't currently happen.  This behavior can also be seen when just splitting a single track when there is a second extremely large one, but in that case the split track still mostly remains on screen; splitting a larger amount gives a much more noticeable effect.

(I ran into this while experimenting with splitting note tracks and then realized that the same behavior applied to the existing stereo track splitting)