Somebody replies to a list message, with To going to a me, and Cc going to the list.
The former has this body:
<x-flowed>
Dave Yost wrote:
> blah blah blah
</x-flowed>
Whereas the latter has this body:
Dave Yost wrote:
> blah blah blah
_______________________________________________
Foo mailing list
foo@bar.com
https://bar.com/cgi-bin/mailman/listinfo/foo
Logged In: YES
user_id=1123998
Originator: NO
Mailman is not doing this unless the original message contains alternative parts and content filtering is removing the part with <x-flowed></x-flowed> tags.
Under some circumstances, Mailman will remove an RFC 3676 Format=Flowed parameter from the Content-Type: header. This is bug #1495122 <https://sourceforge.net/tracker/index.php?func=detail&aid=1495122&group_id=103&atid=100103>.
If some MTA/MDA/MUA is generating the <x-flowed></x-flowed> tags from the Format=Flowed parameter, then Mailman could be indirectly responsible.