Remove all flags, but there's a catch: Tiles can be locked.

When tile is locked, it can't be removed. That requires you to remove the lock that is being held on the tile.

Cheatsheet

This Cheatsheet shows how tiles operate. 

Multiple locks can lock same tile

The reverse,reverses the locked state of the cell, so if cell is locked, it gets unlocked, if cell is unlocked, it gets locked. Vice versa if cell is locked it gets unlocked. When reverse points to each-other, they evaluate that multiple times. Assuming an unlocked cell: First reverse locks it, second reverse does the opposite, and unlocks it.

How reverse operates