How to solve null issue in resources field of service activity?

when I insert a systemuser to Resources look up field on service activity entity save and open it again I find a null value added. I didn't find an explanation about that issue or how to solve it, anyone ?

Solution

This sounds like a MS Product bug and probably limited to a particular version. We can find similar community forum thread discussion on this topic.

You can create a ticket with MS support, they can advise you some hotfix solution patch or workaround.