Frequent test failure on PR builder.

Invalid request submission: {json=[Ljava.lang.String;@2c46358e, .crumb=[Ljava.lang.String;@35661457}
Diagnostics added in 21bf5c45.
Failing I think because message.groovy uses f.submit rather than a plain <input type="submit">,
so JSON gets created and then StaplerRequest.getParameter cannot find it.
But if this is the problem then why would it ever work?
Perhaps a timing issue: JavaScript is used to massage the form just prior to submission,
and maybe this only happens sometimes?
Cannot reproduce test failure locally.