I experienced this bug two times so far. Maybe more often but I didn't notice it.

1st time: I was shaman and next to me one mouse was still alive. The timer did not switch to 20 seconds by itself. (There was certainly no other invisible mouse alive as I could immediately enter the hole after the last mouse)
2nd time: I was shaman on a dual shaman map and both the last mouse and the other shaman entered the hole already. Also here the timer did not switch to 20 seconds.

Both maps were user made maps in the normal map rotation in a normal public room.

Happened again. This time it was in vanilla, the last mouse entered the hole and the timer still showed 00:28
I may be mistaken but I also feel like sometimes it switches to 20 seconds even if there are more than 2 mice left.

What map?

Just now it happened on map 134. Before it happened on user maps, not sure which those were.

Edit: tribehouse testing didn't reproduce the bug

Just to confirm that I am not crazy, and yes I could just enter the hole, there were no invisible players left on the map.