In addition to inserting the { MERGEFIELD Fieldname } field, insert a uniquely named bookmark via Insert->Bookmark, say, immediately after the mergefield field.
I cannot guarantee that that will "work" in all circumstances, but so far it seems to do the trick. Another possibility if you do not want to have to remember any distinct bookmark names would be to insert a nested field like this instead:
{ SET "mfkeeper{ SEQ mfk }" " " }
If you do that, and update the nested SET field before doing anything else (such as updating the HYPERLINK field), you get a uniquely named bookmark (mfkeeper1, etc.). Whether that is sustainable, I cannot currently tell.
This behaviour gpes some way to explaining why the nested HYPERLINK behaviour seems so arbitrary - if you select the HYPERLINK fields and update them all via F9, Word inserts its _GoBack bookmark into the HYPERLINK field, and that's what seems to prevent Word from resolving the nested MERGEFIELD field.
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