I'm having something similar happen to me at the moment. I'm parked at the a gate at Miami Int'l in a non-PMDG aircraft. I'm just sitting there observing VATSIM communications. Every so often I see the scenery around me do the same bumping as you showed in your video. Or perhaps it's the AI airport vehicles themselves doing the up-and-down bumping. I can't quite tell.
I've typically only seen it happen when I switch airplanes while parked at a gate or runway. When the new plane is "plopped" in, everything bounces for a second or two. I've never seen it happen after the initial one. I've also never put myself at this particular airport. And it's my first time joining VATSIM. There are a few things to rule out still.
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Gerard Beekmans