I noticed my cannibal feasts were getting poor quality, even at 100%. It turns out they aren't getting any room bonus.
The ritual start screen displays the full 20% bonus. However, the room's impressiveness is checked at the very end of the cannibal feast. Since the feast generates a massive amount of bloody mess, by the time the feast was done my very impressive dining room was awful.
Thanks for the bug report. I was able to reproduce this in 1.3.3159. I will get a report into the devs for review.
Not a bug, but probably an oversight:
The cannibals feast is one of those "place an object and consume it" rituals that just require the object + room impressiveness (up to 120), but it seems to be the only ritual that makes the room extremely dirty before the ritual is complete, meaning that it's rather hard to get a maximum outcome with this ritual because the blood reduces room impressiveness quickly. For comparison: the smokeleave circle spread a lot of ash from the burning of the bong or the incense shrine, but does so after the ritual is complete. It is otherwise the same ritual in functionality.
Both DLCs, no mods used.