SUMMARY
Not entirely sure if it's a bug, but its behavior is rather unexpected for me.

STEPS TO REPRODUCE
1. Create a layer "A"
2. Create a layer "B"
3. Hide layer "B" (now "A" is visible and "B" is hidden)
4. Select both "A" and "B"
5. Excute the action "Toggle Layer Visibility"

OBSERVED RESULT
"A" and "B" are both visible

EXPECTED RESULT
"A" is hidden and "B" is visible

NOTE
I can see why it works like that, but I believe it shouldn't. It should've toggled all select layers' visiblity.