Per the patch notes:

OMINOUS BOTTLE

-Can be found uncommonly in any Vaults that are unlocked with Trial Keys, and is dropped by Raid Captains when defeated outside a Raid

However, this is not the case.  Vindicator and evoker raid captains do not drop bottles when killed outside a raid, only pillager captains.

To replicate:

• place a villager and composter in a creative world
• drink an ominous bottle
• as soon as raid mobs spawn, break the composter for raid defeat
• wait for raid defeat message to end then kill the captain
• if it was a pillager a bottle will drop, if it was a vindicator, a bottle will not drop
• repeat as necessary to kill both pillager and vindicator captains and observe that only pillager raid captains drop bottles.  You can also wait for a later wave and kill an evoker captain to observe no bottle drops.