OK so I have found the issue here diving a little deeper, however definitely feel like this is not ideal.
When I select manually a post to include and inspect the generated query code, it has ‘post__in’ => array( postID ),
If you allow ‘Select Multiple Values’ in ACF (which I don’t actually want) it obviously returns the post IDs in that same format (within an array) and it works. If you don’t enable multiple values, which I don’t want to, it obviously only returns an ID, not an array, breaking how the query is set up and how the post__in query works.
@Louis would there be a way to check for single or multiple post ID’s and using an array or not? That way it works for everyone. I know this is probably quite tricky possibly but I do feel this could be improved to catch single IDs and insert to an array to prevent it breaking.