i left out one of these:
when you run that zmprov above, is the zimbraPrefGroupMailBy setting for your account set to message? or conversation? or not set? if still set to conversation, this should change it:
zmprov ma [your-account-name] zimbraPrefGroupMailBy message
but before that, could you
turn on debug (which will pop up a window that will show the soap calls), go through the motions of changing it in the web interface again, and attach that soap log here (or pm it to me)