I feel your pain. To solve this, you need to fill in the In-Reply-To header with the message-id of the message you are replying to. I don't use digest mode on the tagging list (I use folders and filters instead), but you should find the message-id somewhere in the message. If it is missing, you can find it online in the archive by clicking on the first link on this page pointing to the sender's e-mail address:
This will open up a message compose window with fields filled in correctly. If your MUA does not support gathering the In-Reply-To value from the link, you can copy&paste it manually from the link after setting it up:
> Date: Sat, 1 Jun 2019 10:01:13 +0200
> From: bkil <bkil.hu+[hidden email]>
> To: "Tag discussion, strategy and related tools"
> <[hidden email]>
> Subject: Re: [Tagging] Tagging buildings that people work in
Thanks for pointing me to the Thunderbird support site.
I get my mailing list messages in digest form. When I want to reply to
a message, I strip out all the other messages from the digest except the
one I'm replying to. Not sure message threading would help me.
Also, thanks to everyone who took the time to send me their comments
about tagging "occupied" buildings.