i don't know if I found a bug but what I noticed is:
The new map preview doesn't work on some maps.
I tried it with the trigger and the f-keys and in singleplayer and multiplayer, but after creating a new preview and going to the map description side to activate the new preview it wasn't there- the map preview just didn't saved or some kind of that. Strange is, that it only doesn't work on some maps..
This could be happening because of other reasons but I just post it here.
(Thanks for answers)