This started happening today.
Repro file: breakingthing.rbxl (12.4 KB)
Motor6D joints are used everywhere in the rig above. Note how the RootJoint has a DesiredAngle set different from the CurrentAngle, so the joint should rotate along its primary axis when the simulation is executed. This works if no animation is executed on the rig itself. However, when an animation is executed on the rig, the joint does not spin at all. CurrentAngle does not change in that case and setting it manually does not have any effect either.
Note how the animation being executed does not contain any reference to RootJoint. RootJoint should spin with Current/DesiredAngle, all other joints should be affected by the animation playing.
In the repro file, simply comment/uncomment the :Play() call in ServerScriptService.Script to observe the issue.
OS: Win 10.