I got it figured out.
Pressing alt indeed allows you to rotate the map.
The alt is "stuck", however, when you alt-tab the game and return to it without alt-tabing, e.g. by minimizing other windows. It rarely happens but it sometimes does in the borderless fullscreen window graphics mode, or whatever it's called properly.
Smashing of the alt key doesn't fix it. If it happens to you, alt-tab off the game and return to it by alt-tabing again.
EDIT
Btw, there are also similar issues connected with window switching. Even though I have the game set to mute sounds when it's off focus, music often keeps playing after I alt-tab it.