I have an underground trading hall/base which is 40-90 blocks below the surface ( under a mountain ). Bringing the bad omen into the trading hall triggers a raid, but the pillagers are unable to get within the village boundaries.
In order to fight the raid, I must leave the boundaries of the village, losing my raid bar, to kill the pillagers. I'm able to proceed through multiple waves, but eventually the raid ends and no victory is declared. Consequently, no hero of the village is awarded.
I've only been able to obtain hero of the village once, and that seemed to require a second player to stay within the village boundary during the entire duration of the raid/waves while I did all the fighting. However, I've tried to reproduce this multiple times with no success.
Since the hero of the village can be used at any village, a suitable temporary work-around is to obtain the effect elsewhere and return to my trading hall. Though, I'm not sure having a transferable HOV effect is working as intended as I've seen multiple bugs discussing this.
Please check if this is still an issue for you in 1.16 Release Candidate 1 or later.