My development computer just unfortunately went to the shop. One thought is @uckelman put some code into MatCargo to give it an automatically-keep-facing-with-Mat, and it’s possible that isn’t accounting for undo by generating an undoable command?
Another quirk in mats behaviour: if you have a cargo on a mat, and you change the mat size by a layer change so that the cargo is no longer on the mat, the cargo continues to be carried by the mat.
Not sure if this is significant enough to merit fixing, but it is counterintuitive.
In my module, I found it useful to be able to shrink mat size to about half size. In an edge case, someone might try to shrink the mat size in order to drop off a unit then move the corps, only to find the dropped off unit continues moving with the corps.
I have some module functionality that could use this in place of a trigger loop. In my case it is just to align a counter to 1 of 6 facings.
This undo bug is still present in 3.6.0 beta 3.