The Special:MassMessage function no longer appears to accept a Page containing list of pages to leave a message on, if the page is in the MassMessageList content model.
This is reproducible on enwiki and test2wiki.
Pages in wikitext format still function.
wikitext (works): https://en.wikipedia.org/wiki/User:Xaosflux/SandboxMMS
wikitext (works): https://test2.wikipedia.org/wiki/User:Xaosflux/SandboxMMS
MML (does not work): https://en.wikipedia.org/wiki/User:Xaosflux/SandboxMMS2
MML (does not work): https://test2.wikipedia.org/wiki/User:Xaosflux/SandboxMMS2