Staff spacer up pushes staff away at a distance
This is a very longstanding issue, that I believe has always been present in MuseScore 2. The issue is simple:
While the staff spacer down only starts to push away a staff below when it is extended until it reaches that staff, the staff spacer up immediately adds a lot of space while still much shorter than the distance between the staves.
The problem is beautifully demonstrated in two scores attached to https://musescore.org/en/node/99661:
https://musescore.org/sites/musescore.org/files/99661.mscz (staff spacer down, correct)
https://musescore.org/sites/musescore.org/files/Example_17.mscz (staff spacer up, incorrect)
I suggest playing with the staff spacer up in the second score to get a sense of how this bug works.
Hopefully fixing this would not present a major compatibility problem.