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?