Dear All - it's not a bug it's a feature, in fact, it's a FIX as far as I'm concerned. Why?
I've been an increasingly heavy email user since the mid-1990s, and having set up and supported many clients with varied email needs. "Conversation mode" has been a nightmare - a blunt tool attempting to compensate for user inability to understand or use 'threading' which email clients has supported since the earliest days, grouping and nesting messages which are genuinely replies to an original message. Unfortunately, people increasingly started to "reply" irrelevantly to messages rather than starting new threads, or failing to reply and breaking up conversations - mostly, to be fair, through poor or non-existent training/explanation of why these things are important.
"Conversation mode" can be seen as an act of desperation, attempting to get email clients to 'reconstruct' some kind of organisation in the worsening blizzard of email. However, 'false positives' are common, especially the grouping by subject which pulls together emails which have nothing to do with each other, and frequently results in important new messages being missed, as they end up 'hidden' inside conversations they have nothing to do with, thanks to subject lines like "Invoice" or "Re: your message". Clients of mine have had to repair business relationships, and quite possibly lost large amounts of business this way.
Of course, I too have gotten used to this 'hack' though and, like others in this thread, will have to find another way to gather together (for example) bounces from my mailing lists and daily security software warning messages which otherwise overwhelm the individual personal messages in my InBoxes. Nonetheless, I am relieved to see a more robust approach - these other methods (particularly filtering into additional folders) have always existed - and I'm looking forward to fewer lost/missed messages (mine and my clients') and therefore fewer desperate/frustrated support calls.
So I repeat - it's not a bug, it's a fix and, while frustrating while we all get used to it, hopefully a reversal out of an unfortunate cul-de-sac in mail client design. Long may "conversation mode" behave more like good old-fashioned threading. Bless all the many people who educate, formally and informally, so that, instead of trying to make software 'idiot-proof' (idiots are too ingenious), we all accept that using any tool benefits from a little learning. So thank you to the thousands of people over the years who have posted instructions on how to use threading and filtering effectively... and indeed on the importance of good 'Subject' lines and correct use of the 'Reply' feature... er, in each case, it's in the name! 😉