Staff spacer down does not work on last system of page
This broke with 3.6.0, but it's only broken with vertical justification enabled.
1) default empty score
2) add spacer down to last staff
3) increase its length
Result: it can be increased until it goes right off the page, with no change to spacing until you go so far that the last system does fit on the page at all any more.
Awkward workaround is to use a vertical frame instead.
This has been a bug that has come and gone over the years, breaking with various different changes to the spacing algorithm. See for instance #281253: Staff spacer down doesn't work at page bottom and my PR that fixed it, https://github.com/musescore/MuseScore/pull/4722 (specifically, the commit https://github.com/musescore/MuseScore/pull/4722/commits/8fa5fcb2d28f5e…).