Fixed milestone move and does not "return" to place

Hello,

when planning with fixed milestone, it will be moved if predecessor is set with end date post milestone date (this can be understood with milestone set to "late").
But, if prédecessor is removed, milestone doesn't return to fixed date.

I Explain :
First, i set a "fixed" milestone
 

Then, this milestone is linked to a predecessor and the date is calculed according to this link (why not, it's a choice, and milestone is set to "late" and show red).
 

After that, link is deleted (i don't want it anymore or linking is a mistake) : but planning date is not changed and milestone doesn't "return" to his place. And i can't change it (planning date is set so milestone is not recalculated...)
 

I can't find a way to set the correct planning date anymore, i need to delete and recreate the milestone or create a new one, fixed, to the right date and link it as predecessor then delete it...

Hello,

strange, i can't reproduce on demo and, after deleting the "wrong" milestone, not able to reproduce anymore on my instance (fresh install V11.1.3).
No explain...