I love recursive code, but I have to tell you that this is recursive in a way the makes my head hurt just a bit.
While I was looking at it and trying to figure out how to make it into a single recursive function that would be easier to follow I noticed that the block_id is a text field and it represents a post ID and I’m pretty sure that it needs to be an integer. It’s a stab in the dark but try changing to this
$target_page = intval(get_sub_field('target_page'));
If that doesn’t do it I’d suggest trying to turn this into a recursive function, there could be a problem with using different template parts.
Also, I don’t see any way that it can stop if the pages reference each other back and forth, which is another reason I would suggest using a recursive function. It would make it easier to add a safety switch, some type of check to make sure you are not looping forever because of what’s been entered.
