Welded objects on the character can be in an incorrect state when an animation rotating the lower torso + anchoring are applied (has to be in that specific order too for some reason). Once this object is in an incorrect state, it seems to apply its weight more heavily onto the character.
This is much more noticeable when you go into shift lock camera where you can consistently make the same move/jumps while keeping the same orientation.
Making the object welded to the character massless will alleviate the issue since it shouldn’t be adding any weight to the characters physics to begin with.
Due to the complicated nature of this bug, I’ve created a repro video + test place to recreate the bug:
(Workaround:)
You can also make this issue go away by toggling the massless property on any part of the character. Seems to trigger an update that fixes the internal issue.
Additional Note:
This bug is being done with only client-side code, it was originally done with server-side code, but it appears it doesn’t matter which one is modifying the player, both trigger the bug.
More specifically, it seems like doing certain actions (welding parts, modifying welds, anchoring parts, changing the mass of parts, etc.) does some calculation for every part. If this part is far from the default position (the position when no animations are playing), the calculation seems to mess up, causing flinging as the humanoid moves around until the calculation is done again. The farther the part is from the default offset, the worse it is. This happens regardless of if the part has mass.
If you play an animation that offsets the character a large amount and force one of the aforementioned actions to happen (such as by equipping a tool), then your limbs will be very far from the default position, causing massive flinging that can manifest as a speed glitch. Note that ending the animation is not required for this to occur.