I don't know whether these two phenomena are connected, but as can be seen in the screenshot below, temlpate FlowMention fails to render in StructuredDiscussions when using the visual editor. The template exists.
Moreover, submitting the topic results in:
[ef03f32d18dfe33cc0c48921] Exception caught: Request to parsoid for "wikitext" to "html" conversion of content connected to title "Topc:Ubege0uwo0i7zyng" failed: 406
Is this related to T191215?
I have this in two wikis running MW 1.30, Flow: REL1_30 9193b03, and Parsoid (0.9.0all) jessie-mediawiki 5fe6923ab