Hello,
Just found this.
When spinning for challenges reward and the bag is chosen (hearing a clicking sound), type /reloadui at that moment.
You will be able to spin again if the bag is not the one you selected !
It's that simple ! So prepare your /reloadui command and if you're not satisfied with the bag, press enter and spin again !
BR,
LDBM
Edit : it doesn't seem to work anyme. I think I had a strange bug.