Fixed in the next build. If the custom BB-Code tag was already set to a unique value, changing the value attempts to rename the tag (a rename only occurs if the tag was created specifically for the prefix). Since the new tag already existed, it is not possible to rename the old tag to it since they would both have the same tag name.
The working way in the current build is to first set the tag to a blank value, Save, then enter the desired value that matches an already-existing tag, as this would avoid trying to rename the prefix-tag.
In the next build, it will check if the new tag already exists when renaming the tag. In this case, it will just point to that existing tag and delete the previous tag.