What can cause a stairs / bumps on runways?

After a good landing the plane suddenly did flips flops in the air.

Poor terrain mesh.

Or over-sensitive and over-reactive suspension.

Mayhap.

Poor terrain mesh is visible. Is this specific to a runway?

I am getting it at random times and places
Yesterday it was very bad for example at Baltimore-Washington Intl (KBWI) on runway 28

Open a support ticket
https://flightsimulator.zendesk.com/hc/en-us/requests/new

I will try that runway, but it might help to know which plane also. Happens with all planes or no?

yes , mostly I fly with the FBW and PMDG now and it happen on both on them.
And it happen random

Yep me too, on Xbox not every time but sometimes that happen, I’m really sad this sim was awesome before su9, I hope it will be fixed soon, a lot of problems now… really sad…

Poor terrain loading due to slow internet connection. Lost connection to MSFS server. Commonly happens at generic airports. I haven’t seen it at payware airports maybe because that is a set loaded terrain file in the program. Best answer I can give.

Yeah that coult be it…
I have fiber optics 1000/100mb , so I hope microsoft will improve their servers…