Wooo! Got it working.
Well, eventually the lightbulb clicked - I don’t need to enable the user in the Function. I can call the BO directly, since I know that works.
So, it needs some polish, and I’ll write it up in a separate post here, but this works in 2021.2 and 2022.2:
As for the original (actual) issue, I still don’t get why the update fails in any kind of function I made. Sorry @Devin.Draeger
Since yesterday, I have tried Ice vs Erp, Update vs UpdateExt, and a dozen other things. But if they don’t work and all I need is to do one more thing, I’ll just skip the Function for that part.