Summary:
The hostile mobs associated with the raid aren't going to the village center? they are just stopping outside of it (which I guess is a bug). And never going towards the village center. I assume this is a AI / path finding problem.
What I expected to happen:
I'd expect all the hostile mobs associated with the raid to run towards the villagers, which is placed in the center of the village (shown on the pictures).
What actually happened:
All the hostile mobs associated with the raid are stopping outside the village - and never path finding their way to the village center (where the villagers are located).
How to re-produce this bug?
Build a setup like shown on the pictures. And then trigger a raid with the bad omen effect.
Please check if this is still an issue for you in 1.16 Release Candidate 1 or later.