I just checked on the group layout in our own Lithium Community here and a group message's title is actually represented as <h1> within the PageTitle component.
What you can verify on your end:
In Lithium Studio on your stage community: Verify your GroupTopicHeader layout contains the PageTitle component and not any older - non-responsive - widgets like message-view.widget.subject
Within Admin -> System -> SEO check that the appropriate tag is selected for subjects on message lists and on forums. Admin>System>SEO setting for subject tag NB: Most of the time the default setting of h2 is actually the correct one since h1 is already set on these pages for the board name or widget title. So please use caution with changing this to h1. A very easy way to verify is using the "HeadingsMap" browser extension (Firefox, Chrome) which shows
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