I have sales leads come in via email which creates a SharePoint list item. Via different statuses (approve, reject, etc) the list can trigger different emails to go out to the lead.
When the lead responds, I don't want to create a new item, but rather just update the existing list item. Using the ConversationID I have got it so that the response comes in fine and has "Update" all over it to help folks realize it's related. Like you I used Get Items, filtered for my ConversationID and then if it matched, told the system to update the existing SharePoint item. The problem is the ID parameter seems to cause SharePoint to create a new Sharepoint Item rather than UPDATE the existing one.
I've done Update Item flows a thousand times, but am really stumped this week - must be a case of Monday fog!!!