[Users] "Thread using subject in addition to standard headers" not working?

[Paul]

On Mon, 11 Jan 2021 14:17:14 +0100
Thomas Leitner <t_leitner at gmx.at> wrote: 

> This thread works because there *is* an In-Reply-To header as well as a
> References header for all messages after the original one (which
> obviously can't have such headers).

Yes, so what's different about the your problem messages?
In other words, you have not provided enough information about your query to
be able to diagnose it.

with regards

Paul