Hmm. I'd expect likewise Randy.
Tegwyn, starting closer to home, I suggest you try this first, which I'd expect (messily) to bring back the ID of the primary assignee (from the Task, mind you) as a suffix behind each of the Task names:
IF({numberOfChildren}=0,IF({canStart},IFIN({status},"CPL","FLG","",CONCAT("• ",{name}," ",{assignedToID}," "))))
If that then works, one more baby step would be to try to hit the assignedTo object itself and return the (very same) ID:
IF({numberOfChildren}=0,IF({canStart},IFIN({status},"CPL","FLG","",CONCAT("• ",{name}," ",{assignedTo}.{ID}," "))))
If that (syntax) then works, like Randy, I'd expect his previous suggestion to work (but we know it already fails), so perhaps it is not "reachable", being in a collection (e.g. too many hops). If so (even if there is some elusive syntax that would do the trick), as a workaround, if you happen to already have a custom form on every Task of interest, you could add a calculated parameter (e.g. "Current Primary Assignee") with a formula of "Assigned To:Name" on each task, and then, having brought what you need "closer" (kinda) on the Task, try this:
IF({numberOfChildren}=0,IF({canStart},IFIN({status},"CPL","FLG","",CONCAT("• ",{name}," ",{DE:Current Primary Assignee}," "))))
Good luck: I'm interested to hear how you make out.
Regards,
Doug