What kind of ‘bug’ are you referring to? In fact, there is behaviour documented in 22.6 Patch Notes:
You will now be able to set different Equipment and Portrait loadouts for your different Parties using the same Mercenary; you will no longer be able to change these loadouts in the middle of a Bounty.
What that means in practice is that, if you just change the equipment somewhere in the collection manager, it will not affect existing parties with their own equipment saved, which, by the way, is rather convenient and has been much desired and requested by the community, as far as I recall.
PS What you want to do is to equip the item you want for the party that you are going with to a bounty (or a pit fight). If you just equip an item in the collection manager, then it will serve as a ‘default’ equipment for any new parties you might make; however, unless they’ve changed something and I’ve missed it, if you later switch the equipment even while editing an existing party, then this new item will become the new ‘pre-equipped’ default.