As reported initially by @kichae@lemmy.ca, there are some posts from Lemmy that didn't make it over to NodeBB. This turned out to be an issue with how content was parsed by NodeBB if source.content was also present.
The code has been updated so that if a content source of type text/markdown is provided, then that source is used instead of the generated HTML.
Outside of Lemmy, this also applies to any implementor (snac2 maybe?) that also implements the source.content property.