Hi Tom,
Thanks for your response. Looking at #2 and #3 again and testing in my test course (Moodle 2.6+), I'm seeing that the submit button in both cases is below Edit submission. What do you actually see down there where the submit assignment button should be?
My first thought is that the html is getting truncated as it feeds to the browser. I'd be inclined to view source for a page returned with "submit assignment" and one without and compare. If they are different (they probably are), the question becomes "why is the html being truncated?"
There is a similar tracking item here https://tracker.moodle.org/browse/MDL-44099?jql=text%20~%20%22submit%20button%22 with a screencast, but I think that case is confused by the browser having both a student and a teacher logged in at the same time.