Never seen this happen with the stock Jetway program with Inverse Kinematics.
No. Even the stock jetways have the same problem. It might not show with the same airplane but, for example, if you try the default CRJ over a default scenery, the jetways sinks into the ground too.
it seems to me that the wheels where not lock to the ground as shown in the video and that the animation to raise and lower the Jetway on the two shafts is not operating.
The jetways are modeled correctly as per MS specs. That issue is an FSX bug which, as I've said, happens also with the default one. The exact behaviour with different airplanes changes, but the problem is present also in the default jetways.