Why don't all the letters fall off when i complete a word on alphabetty?

Cheri_Basl

Why don't all the letters disappear when I complete a word on the game of Alphabetty?

StuckInATree

If you use any tiles that are covered in vines they will not fall off the board until the next time you use them in a word. The reason is the first time you use them it only removes the vines off them, only then when they are used again that they fall off.