Here’s my idea on what’s going on, and how to (probably) fix it:
This is a quick “elbow rig” I made. However…
Yikes! Looks like Orange has melted a bit. Normally, we’d have to use Weight Painting to sort this out, but since we’re working with Roblox’s segmented rigid parts instead of something like a skinned, stretchy smooth rig, we can solve this easily.
Go into
Edit Mode onto the melted part, and then in the Properties window, go to the Vertex Tab as shown on the right.
Remove any Vertex Groups that don’t belong to the part, such as “Blue Bone” here, via selecting the group, and then clicking the minus button.
To make sure that ONLY its intended bone controls all of it, select the Vertex Group of its intended bone, set the Weight to 1 in the same window, and press A to select all of the vertices of your melted mesh.
Then, click “Assign” in Vertex Groups.
Use Pose Mode to see if it works now!
(NOTE: A Vertex Group’s name MUST EXACTLY match whatever bone it’s referring to.)