Is fork dipping when front brakes applied at very slow speed normal?

 
Sounds very similar to what I’m experiencing!
When coming to a slow stop my forks dip and return to a normal position rapidly with very little movement in the break lever, on a downgrade it intensifies. Had my wife watch as I applied the front break and the dip happens at the same location on the rotor using the valve stem as a marker, “ it’s easier to do this on a downgrade as no throttle is required and at a walking phase.” Once past the rotor the forks rise to a normal position. So to me it appears to be a warped rotor,
Did a lot of searching and to my surprise no one mentions the rotor as being the culprit.
The bike is new to me with 18000 kilometers,
I’m getting the rotor tested for runout in a few days and I’m betting it’s the rotor. Also no rotor is available in town and will have to be ordered, I’ll post the results of test and fix when available.