After killing off the second wave in a Normal Mode raid with bad omen II (Should have had at least 5 waves) the boss bar for the third wave loaded and then disappeared without any mobs spawning or the "Ominous Horn Blares" sound effect. 

EDIT: I have added a world download for the same build just in a creative world. 

Steps to Reproduce The Bug; 

1. Start at coordinates 1, 80, 1 this should be your spawn point and is a model of my pillager tower farm. 
2. Check your game mode and then give yourself the bad omen effect with the /give command.
3. Follow the nether portal at 1, 80, 9 into the nether
4. Once in the nether, you should see another portal close by, Go into it and that should lead to the village location where you will start a raid.
5. Kill the raid party on the floor above the portal where the hoppers are.
6. (Maybe) Encounter the bug! The bug usually happens around waves 1-3. I do not encounter the bug every time since it seems to be random but I have a video attached below of the bug happening in that world specifically.

Some tips: You cannot give yourself the bad omen effect when you are already at the village. You need to give yourself the effect before traveling from the nether to the village since that will change where the game thinks the "Center of the village" is and the mobs will not pathfind correctly. Also, if you fly up to watch the raiding party spawn you will be out of the range of the village and will not be able to witness the boss bar disappearing. 

Data after some testing in my survival world

1. Wave 3 Bug
2. Wave 4 Bug
3. Wave 7 Complete Raid(No bug)
4. Wave 5 Complete Raid
5. Wave 4 Bug
6. Wave 5 Complete Raid
7. Bug loading the first wave
8. Wave 3 Bug