I am having the same problem.
Given an email that was dragged from mailbird some time ago and that has responses that have added Re: to the subject line
When you double click on them in the mailbird Legend App
Then they are not found (Legend doesn’t say anything just doesn’t react)
New emails being just dragged work fine, when double clicking them the proper email is found in mailbird and displayed.
In my case it’s particular important because my workflow transfers all my action items from my mailboxES to Legend, so my mailboxES are just a repository of old emails but all my action items are in Legend. I use gmail, outlook and 2 other emails and that’s why mailbird is crucial for my work, as wrangling all my email with browser interfaces would be a good cause for slitting my wrist or somebody’s neck. ha ha
My use case is giving follow up to my co workers pending items so being able to quickly double click on a work item and pointing to the email so that I can reply is crucial for productive work.
I am raising the priority of this becuase of that.