Both brawlers and (deg) items work the same way when degrading. Allow me to explain with a simple example of brawlers.
You own 2 prayer brawlers, one is at 500 charges. You bury 125 bones while wearing one pair, switch pairs and bury another 125. Right clicking the brawlers will tell you you have 250 charges left. No matter which brawlers you wear and check. This is because these values are stored apart from the item. Once this value reaches 0 one brawler/deg item will dissapear and it will be full again. Repairing ancient items will just refill the charges. This is all intentional, no bugs here whatsoever.