In T145282#5072227, @DannyS712 wrote:

@Base that limitation isn't based on the mass message extension. Likely, the reason is that the maximum section name limit is applied before the wikitext would be parsed. I copied and pasted the section heading at https://en.wikipedia.beta.wmflabs.org/w/index.php?title=User_talk%3ADannyS712&type=revision&diff=391441&oldid=391395, and it also got cut off.

Indeed, so this isn't a MassMessage bug but MediaWiki core design; closing as invalid for clarity as long a the report title is phrased this way.

There is a case to be made for a feature request of the kind "Allow MassMessage to carry template calls bigger than size limits" (which IMHO is not a very good idea), but that's not a very good idea because we cannot know how a certain wikitext will end up being parsed in a local wiki: the workaround is to create a template locally to host your desired content, which MassMessage can subst; creating many local templates of course is not especially nice either, but we end up in T6547 territory.

I think the way to go is to formulate this as some feature requests to improve the sending of a translatable page.