First, I looked up the ListItemsEditForm property. It says nothing about having an automatic OpenArgs tie-in. So if there WAS an open args entry, it was done manually and that means from the call that invoked the form.
However, you DO NOT CARE which form called which form. Look up the two events called _Activate and _Deactivate - which come into play when focus changes from one form to another for any reason. When focus re-enters a form, do the requery in the (still open) calling form's Form_Activate event. Let the closing form (Form C in the example) just go back to sleep.
You were trying to make form C do something nice for the form that opened it but it is a pain in the patootie to actually determine (via system calls) which form called you, and yet the suggestion to pass that information via OpenArgs seems to be met with some friction. The easiest choice is to have that calling form do its own requery, which you can do from a Form_Activate event.
Office VBA reference topic
learn.microsoft.com
Excerpt from that link: "When you switch between two open forms, the
Deactivate event occurs for the form being switched from, and the
Activate event occurs for the form being switched to."
So you DO NOT CARE how form C got called and you don't care whether A or B called C or you ran C directly for some reason. Put the
combobox.Requery in the _Activate event for each of A and B. At worst, this causes 1 extra .Requery when you open the form for its first use. AND if you were to directly open C (for testing, e.g.) it doesn't have to look for something that wasn't there in the first place.