Loop with ACF relationship field & Orderby "post__in"

First up, I noticed that your tags are written in lower case, which isn’t correct. You mention that you’ve got it working so I assume that’s just some side-effect of your code getting processed by Discourse. Pro-tip: when you’re writing code, be sure to wrap it in backticks, like this ` (or use the code option in the post editor) if you want to write inline code. I’ve modified your post to make your tags visible.

Regarding your issue, I think it might be similar to this thread. Basically, I don’t think that ACF relationship loops accept query parameters like the post loop does. I imagine it might be possible to add this feature, but that’s at least how it works right now. So the approach I suggested in that other thread is the same one you came up with.

I think that’s correct. I’m seeing a feature request here related to this that hadn’t been released yet.