I’m a new user of Tiled and i can’t work with terrain tools.
The first time when i used the Tiled after installation it works, but after this time is happining this.
In one click, I dragged the mouse in the format of a 8 and it happeneed.
How fix it?
This happens because you only marked the outside corners and forgot the inside corner tiles right above them. In effect, the terrain tool does not have inside corners at its disposal. I think it performed quite well given the circumstances! 
so simple… thank you