Single line staff is spaced as if five lines
S5 - Suggestion
If a part is on a one-line staff, you would expect things that are spaced to be above the top or below the bottom of the staff to adjust to those two places being on the same line. But MuseScore treats it as if there are four more invisible lines below a visible top one. This looks slightly strange in a score with the apparent extra space below the one staff among many, but it becomes a real problem in the part itself:
Is it too late?
EDIT: That one mezzopiano at the top that's looks like it's in the right place is because I manually moved to the right place. Just so you know.
Markings in MuseScore are positioned relative to the top staff line, as specified by the text or general style parameter that controls the position of that element. If you have fewer lines then the default, and you add elements that are positioned below the staff, then it is up to you to position these elements appropriately relative to that top line. Eg, 8sp is an appropriate distance relative to the top line for dynamics, and hence it is the default. If you create a staff with fewer lines, 8sp is no longer appropriate. So you will need to manually move dynamics up on that staff (eg, right click one, select all similar, adjust vertical offset in Inspector).
So what you are describing is not a bug but just the way it is / has to be given the current design. What I think you actually want is a new feature that would allow positions to be specific specified relative to the bottonm line instead of top line. I think that sort of thing would go along with the desire to have separate above staff & below staff settings in general, also the ability to override style settings on a per-staff basis. This is the sort of thing I'd like to see in 2.1 if possible.
Came up in https://musescore.org/en/node/109461 again