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.
Seems like Roblox reverted this change. The issue is now once again only happening to gear which doesn’t include a ThumbnailPose.
Most gear created in 2017 and onward include a ThumbnailPose. Older gear probably don’t, unless Roblox updated the specific gear item at some point within the last few years.
This happened as I was equipping something for my avatar, It then broke the Idle and I couldn’t seem to get it back, there was when I found out It was a bug.
This no longer happens due to the release of Roblox’s profile pose feature. The 3D view still breaks with gear and layered clothing, but that is a separate (possibly related) bug.