WTFF

Nevermind, I found a fix so I'll publish it here, since it seems like no one has explained it clearly. There may be many other ways, but this is what worked for me: 1- Set all the draugr and alcove/coffin markers as initially disabled. 2- Place a "defaultActivateSelfTRIG" at the cell entrance, and on the scripts tab remove its default script. 3- Click "Add", "[New Script]", name it however you like, and paste the following: Remove/add as many instances of "Draugr#" and "Ambush#" as needed for your dungengeon and number accordingly. - Save the script. Double click and edit each property picking in the render window the draugr (for "Draugr#") and markers (for "Ambush#") you previously disabled. - Save and done. They will now spawn inside their coffins and start the ambush package properly.

Sorry for the necro, but I'm having this same issue and nothing has worked so far. The ambushes work properly when I COC from the main menu, but when I went to test it in-game, every draugr would stand outside the sarcophagus. I've tried loading my first saves, reinstalling the mod, waiting for the cells to refresh, using all the options "save cleaner" provides, removing skyrim.ini and every possible combination of the previous... no result whatsoever. I've also tried what has been said in the previous comment, and still the same. No other ideas?

Sorry for the necro, but I'll write the answer in case someone is looking for it, since I've just been messing around with this for a unique NPC. You need to edit the "DragonPriestActorScript", you can find it in the "AbDragonPriestScriptAttach" object. Right click the script and "Edit Source". Copy everything, close the window, then Add>New script, paste, then Scroll down to "EVENT OnDying(Actor akKiller)" and delete the ash pile related lines. To prevent any issues, save with a different name, set every variable as the original, and remove the original "DragonPriestActorScript" from "AbDragonPriestScriptAttach", so that if something goes wrong the default is still intact.