I was having this problem too, I think roblox ““fixed”” bounding boxes by making them all the same and not announcing it, they have done several changes with their introduction of PBR-enabled UGC uploads which I guess would affect the upload validation.
The only solution I can suggest to you all is that you upload the UGC shifted up to fit in the bounding box and have players correct that with the Accessory Adjustment tool, I recommend you investigate how much you can move an accessory. If your UGC is still out of bounding box (if you do something like gloves that reaches below the hand it likely will) I also recommend you scale it down to 83.33%, so that people can size it to 120% (the maximum in the Accessory Adjustment).
Below are examples of how I’ve done it, the arms were scaled to 83.33% and moved up 0.475 studs, making them barely fit:
I hope this helps you, it took me multiple uploads to figure it out, great way to spend money 