This behavior was noticed on a realms server where I was using a raid farm and received bad omen while there was an active raid.

Further testing on a local world shows that the effect acted as expected as long as the player didn't receive bad omen while in an active raid. However, if the player in currently in a raid when they get the effect then it immediately disappears, but will still trigger a raid if the player enters a new village.

Both drinking Milk and "/effect @p clear" have no effect.

Video linked below.

https://drive.google.com/file/d/1w6Yyq0I6o2TcMuNFzcZUFDRlguudZnro/view?usp=drive_link