Task constraints are automatically changing

When I have a constraint on a task, e.g. “snet 1 January”, and I move my task to 7 January, then this should be a valid move. However, the constraint itself also changes.

This behaviour seems to be documented here:

" When a user changes the date of a task by moving it with the mouse pointer or via the lightbox, the task automatically receives one of the two constraint types: either start no earlier than+%start date% or finish no later than+%end date%, depending on the chosen planning strategy."

How do I prevent constraints from changing?