Undoing system breaks doesn't always restore measure spacing to default.
(The Layout type is missing from the dropdown menu so I picked Graphical.)
When placing system breaks, the allocation of measures to each system doesn't stay the same as before even when you hit undo. In the full score, this can get fixed when saving, but in parts you need to close and reopen the file for it to fix.
In the given score, try placing a break on measure 12 and removing it.
My apologies if this has already been submitted, I looked for it but haven't found it.