I just updated from 68.12.1 (32-bit) to 78.4.0 (32-bit) and I don'tI've done some testing and it seems to relate to the <a> element wrapped around the mailto link:
<a target=3D"_blank" href=3D"...">Reply To Group</a>
In particular, the target="_blank" attribute seems to be the troublemaker. Absent that the enclosed mailto: seems to work fine, and opens a new composition window.
That still makes it a Thunderbird bug, unless there's some reason one isn't supposed to combine target with a mailto: protocol in the href. Which seems like an unlikely restriction. But it may explain how it got through testing if that's an unusual construction.
(For those following along at home, the "=3D" sequences in the example are just equal signs, in quoted-printable form. A consequence of me copying from a view source of the HTML message body.)