From what I have seen this happens every single time you disconnect or go AFK in the tower and are removed from the game.
You will be removed from the game without leaving the tower which ultimately means you lose all of your rewards from the tower. I lost out on 30 floors of my first tower clear cause I went AFK and when I came back it said the floors were completed but the rewards I never received.
From what I know this happens every single time. I would really like my rewards please and hopefully this can be fixed so noone else accidentaly misses out.