Tl;dr: Opened woven backpacks no work in soulbound backpack recipe. Fix plz. When a woven backpack is opened, a tag appears with it, that tells the player which backpack it is (a unique ID, which acts as a tag so the server knows what the contents are). The recipe for the soulbound backpack calls for a woven backpack without this ID present, and I believe this causes any woven backpacks that have been opened to not be able to be used in said recipe. If it would be possible for players to reset a backpack's tag, or for the recipe to allow any woven backpack to be used, that would be very useful. Or, if it would be possible for woven backpacks to reset themselves (turn into a backpack as if it were just bought) whenever there are no items present inside the backpack, and re-use that ID when opening or containing items inside, that could fix this problem, and prevent players from losing items when using a woven backpack in the recipe. However, this fix would cause more lag when opening backpacks, not making it entirely ideal, despite being easier to implement.
If I may offer a suggestion. Have a Traders in spawn. Have the item you want to have repaired in a sense like making a fresh woven backpack, have it cost (two item slot trade for one Item.) like the backpack and 5 emeralds to get a brand new one.