Please try to replicate (is that a Bug?)
- In Write mode create a rehearsal mark
- attached to same bar, create a time equation
- In Engrave mode try to move one of them in any direction
Result: both move up away from staff.
try to undo, they keep moving far away.
if you have enough patience, keep undoing, at some point the time equation will be erased (undo works at last) und you are back at the beginning.
The alternative of course is to enter the time equation as system text.
Edit: easier alternative, attached the time equation to a different point in the rythmic grid.