For some reason the new ominous bottles are stackable and also (clean/empty/regular) bottles and tipped arrows, but water bottles and every potion isn't. It feels so weird as it was a bug, probably is and MS knows it, but it's time to fix it. That would fit to the new 1.21 update which has "tinkering and combat adventures" theme, so the stackable potions fix fits as well and also it would fix some inventory issues.