In the below save, my colony's moral leader, Herring, is standing near some alpacas (I drafted her and made her walk there), and currently has the work queue of "Hauling chemfuel x11 ..." followed by "Making large sculpture".
I go to the altar and activate the "Begin Supreme Sacrifice" (animal sacrifice) ritual. The default of Alpaca 15 works for this; it doesn't seem to matter which animal.
Most colonists drop what they're doing and go to the ritual. But Herring persists with hauling the chemfuel, then works at the art bench indefinitely, while everyone else sits in the ritual room waiting—note the 3-hour counter doesn't start until the animal is brought in. I can't tell Herring to snap out of it by drafting and undrafting her, because it's during a ritual. As a player, I assume all my colonists are stuck until she finishes the statue or I cancel the ritual (which I've never done before, and assume has negative consequences given the confirmation popup), and prepare to file a bug report.
As it turns out, Herring "only" works at the art bench for roughly 1 in-game hour before dropping it and walking back to the alpacas to bring in the sacrifice. Also, it seems there are no negative consequences (other than any already-wasted time) to cancelling the event, and one can restart it immediately. (Also I could have stopped the art work by forbidding the sculpture; the hauling I could have made her drop by temporarily deleting the target square of the stockpile; but a player shouldn't have to invent these workarounds.) So it's not as bad for a sophisticated player as it naively seemed; still, it seems illogical and a bad experience.
I've tried some variations on this. I used shift-click to add a third item to Herring's queue: butchering, and that remained in the work queue after I started the ritual, although I did have the ability to cancel it with the C key (it cancels the butchering but not the rest). I also tried giving a random participant colonist a sequence of orders (cleaning, hauling far-away objects, stripping a corpse); it seems that activating the ritual cancels some but not all of the orders, and the remaining ones can be canceled with the C key (after which he goes to spectate). I've also managed to get a spectator colonist to instead haul the items off of a stove (he's a cook), in a non-cancelable way, when he should be spectating.
Also, if I do e.g. a public execution instead of an animal sacrifice (with Ito as executioner), Herring is still going to haul the chemfuel and then sit at the art bench. Probably this happens with all rituals.
I'm not sure exactly how the existing rules behave, or what would be ideal. But I think the intent is supposed to be "during a ritual, colonists shouldn't be doing anything non-ritual"; the fact that you can't give them new orders during a ritual strongly implies that. In that case, it should be that colonists always drop everything they're doing (maybe with a small list of exceptions, like eating food? maybe no exceptions) to attend the ritual. It seems that that's usually what happens, but sometimes it's not, and this seems like a bug.
https://www92.zippyshare.com/v/NLsxLt9X/file.html
Thanks for the bug report. I was able to reproduce this on your save, though keep in mind normally we do not accept bug reports with modded saves. Even if its only one mod (though I have heard the non-author updates of eDB Prepare Carefully are not the most stable, I'd recommend trying something different and actively supported like Character Editor.) I will need to see if I can reproduce it on a completely vanilla save, then I'll get a bug report into the devs.