I went to a village with bad omen to trigger a raid, and bad omen did become raid omen upon entering the village, but when raid omen dropped 30 seconds later, the raid bar didn’t show up. I had to leave the village, get bad omen again and go back to the village again. This time it worked but I still had raid omen active (and about to drop again) just as the raid actually started.

To reproduce: 

1- get bad omen 

2- enter village 

3- wait for something to happen 

4- if nothing happens, leave village, get bad omen again, then go back to village and try again