I googled around a little bit, and did some simple mailing-replying
actions.. It seems like gmail uses "subject" for threading the
conversations. Kmail uses something else (i guess, In-Reply-To). Not
just that, when you change the subject line when replying to a
thread/conversation using gmail, it "removes" the In-Reply-To header;
as a consequence any MUA ( i think) will get that messsage wrongly
threaded. So, to me it seems like a problem with gmail's mailing
system rather than MUA at the reading end... To prove the point
(hopefully!), my next mail on this thread (using gmail) will have a
modified subject line, check if the In-Reply-To header is removed, or
whether it is threaded in a proper way with whichever MUA you use at
your end..