There is a function to make a linked between incident and problem records.
When a problem record is linked to multiple incidents, only the last incident attached to the problem will have the link.
The problem was removed from the related list of the initial incident(s) and added to the last incident only.
The Problem should be visible in the related list of all the incidents.
On the related list "Problems" on the incident form, we are actually setting the Problem's parent to that incident.
Since the "parent" on the Problem is a one to one relationship reference, it only allows one parent per problem so that is the expected behavior.