Systems can overlap (by same amount that staves *don't*)
When there are text and notes above one system and also below the system above it, the systems may overlap and auto avoidance does not take this into consideration as can be seen in this picture:
which was taken of mm 62 & 68 of Beethoven String Quartet Op 131.mscz which is included for examination. The only changes I made to the default laoyout were to the Y offset for dynamics and hairpins (I reduced them because I think the default puts them too far from the staff).
You can of course change settings to prevent such overlaps, but the problem is that auto avoidance does well in adjusting staff distance, it fails in adjusting system distance.