The 15th would never be spotted "behind" the rack, unless the rack has been disturbed and a ball interferes with the foot spot.
If the 15th ball is called and made or missed, on a legal shot, without the rack being disturbed, then if both players
agree that the 14 balls in the rack needs to be adjusted... OK. Again, check the rack before the shot
If the 15th ball is "made in the wrong pocket", "made on a called safety" or "made in called pocket with cue ball scratch"
or just a "cue ball scratch" then a "different" rule comes into play in that the full 15 ball rack is allowed to be "tightened up"
by re-racking all 15 balls if the 15th ball will not freeze to the rack. This can be "done" by either player without approval
or complaint from opponent. In fact, the players in our 14.1 league generally rerack all 15 balls in this scenario without thinking
about it or trying to freeze the 15th ball to the rack.