I found a weird behavior -- I don't know if it's worthy of an issue or not, but here it is:
The tempo marking will not avoid a slur that goes from staff to staff. Here is a snippet that shows the problem, and I've attached an image of it: \version "2.25.12" one = \relative { \tempo "Allegro maestoso" c'''2( e | \change Staff = "lower" c,1) | } two = \relative { s1 | s1 | } << \new Staff = "upper" \one \new Staff = "lower" \two >> -- Knute Snortum