This isn't a bug but just the way it works here, same with dragon amulets etc. Once the charges have been used up one ring will be destroyed and then the count will start again.

I understand that the one item should have the charges used up. However, if I have a bank with 8 rings, all with 8 charges. I take one out and use it. When I go back to my bank, all 8 of my rings now have 7 charges despite only using one ring once. It's frustrating because every new ring I get now has 3 charges when I click on my bank (even if I don't deposit it). This prohibits users from having multiple slayer rings as the amount of charges they have will be replaced with the one that's lowest.

It's the same with dragon & slayer amulets, they are locked to your accounts usage. All the other ammys/rings have the same 4/8 uses they would do when fully charged/new. When you examine a ring or amulet, it only shows how many charges you have left before that ring or amulet is used up. There's no bug mate, just the way it's coded. If you were to trade a ring or ammy that only had one charge left to a new level 3 account, that ring or ammy would also have 4/8 uses.