You would have to provide more details before we could analyze the specific situation to determine whether those things I mentioned are not coming into play.
Split Level can force unRAID to put a file on a disk other than the new disk. And 0 Minimum Free is not recommended and is probably one of the main reasons people get unexpected out-of-space. unRAID has no way to know how large a file will become when it chooses a disk to write. If a disk has less than Minimum Free, it will choose a different disk. You should set Minimum Free to larger than the largest file you expect to write to the User Share. There is also a Minimum Free setting for cache in Global Share Settings.
But as I mentioned, Split Level takes precedence, so if Split Level says a file should go on a certain disk to keep it together with other files, then that is the disk it is going to choose.