So i was using Kiwi and i dind't have the 5th inventory slot for legendary's and swapped it with something else. but my new tower dind't get his own 5th slot that i normally do have. using the GT-1 and the plasma blade gave me the 5th slot back and 6th bc of the passive.
I can confirm this bug. When swapping a tower that has 5 slots to a tower that has 4, then back to a tower that should have 5, the new tower will only have 4. If you sell the tower and then build a new one, it will have the correct number of slots.