See the gif below:
Notice how, directly after "stay"(the flag skill) is selected for the first time, it vanishes? This bug happens in both castle and sandbox mode. The skill is unusable until it reappears. The skill reappears when you unlock any other skill.
In the gif, the player unlocks the flag skill. It disappears, and then they unlock the hammer, causing the flag to appear on the right side of the image.
This bug always occurs when you unlock stay. It doesn't matter if you've already unlocked other skills. This is a pretty annoying bug that needs fixing.
Thanks to Chikkuri for the gif
Customer support service by UserEcho