This doesn’t happen with all gear, just gear that has a Keyframe named “ThumbnailPose” inside the Tool object.
A little over a week ago they finally fixed the years-long issue where gear without a custom thumbnail pose would reset R15 characters to a default un-animated state in avatar thumbnails. For some reason, gear that does have a custom thumbnail pose is now experiencing this issue. (but the gear’s thumbnail pose still works)
Note: Most gear with a custom thumbnail pose just animated the right arm to appear outstretched, nothing fancy. This allowed the gear item to be properly held with your R15 idle pose, before Roblox finally added support for that.
I’m currently experiencing a similar issue today. Instead of my right arm holding out the gear in sync with my currently equipped idle animation, it became stiff and did the same pose (old pose) as after the issue started occurring on the 15th.
I also realized a related issue on the 6th of this month when different gears that were absolutely in-sync with your idle animation (no right arm performances) were fixed to display right arm performance, however all of the other gears that were already fixed beforehand were now broken and display stiff animations.