Whenever I apply idle animations to a character who is sitting down, I get a glitch where the NPC can never sit but stands up in their chair. This is a documented bug in Knights, as per UESP:

"After returning to the Priory from the final battle with Umaril, the player may encounter a bug where they lose the ability to sit; either no sitting animation plays and the player appears to be standing but can't move, or the player sits on nothing beside the chair/bench. The cause of this is an idle animation with bad conditions that "sticks" to the player. There is no known way of fixing this with console commands, but it may go away by itself after extended gameplay when many other animations have been applied to the player."

ALSO I have found that every NPC in the game (including the player) can no longer sit.

What is this caused by? How can I fix it?