Rig Deformation: Mesh Deforms Badly When In Pose Mode - When No Pose Changes Have Been Made I'm still pretty new to modeling and rigging, so apologies if this question seems obvious. But I tried to apply a skeleton and rig to a mesh I was working on. Things seemed to be fine, until I switched the rig from Rest Position to Pose Position: before I had even made any changes to the rig, it deformed the mesh heavily. The face seems to have been hit the hardest, sagging and being pulled forward around the mouth and eyes.

If it helps clarify things, I set the rig as the mesh's parent using the following path: &quot;Parent&gt;Armature Deform&gt;With Automatic Weights&quot;. This was the Armature Deform option which seemed to be causing issues with mesh deformation, but it also seemed to be the mode which lent itself to posing most easily (the two are likely related, but I don't know how to troubleshoot this).
Do you have any troubleshoot options I can try to fix this, or can you otherwise recognize what the issue is? I'll attach the Blender model here for reference, in case you can find the issue here.
to save space, the model has been compressed into a .zip file
It's because of the bone called DEF-chin.R (here in Rest Position):

you'll be able to see it if you enable its visibility in the armature layers:

You've given it a Damped Track and a Track To constraint with the bone called lips.R as Target, so as soon as you switch your armature to Pose Position it will point towards it:

When you parent a mesh it takes the armature Rest Position into account, and not its Pose Position, so when your armature switch to Pose Position, the bone will automatically move or rotate towards its target.
So you need to make sure that the position of this bone in Edit mode (i.e. their position in Rest Position) is the same position and rotation as in Pose mode, and reparent.
In Pose mode you can go in the header menu &gt; Pose &gt; Apply &gt; Apply Pose as Rest Pose. Make sure to duplicate your armature first so you can come back to a previous version.