Go to https://en.wikipedia.org/wiki/User:Neil_P._Quinn/Testcase_1?action=edit.
Reuse citation 1 at the end of the first paragraph. Then, delete the reflist template and replace it with a normal references list.
Now, review your edit. The original citation 1 has disappeared, leaving only the reused citation 1, without any content.
Seen in Firefox 51.0b4.
This just happened to me! https://en.wikipedia.org/w/index.php?title=Monoplegia&diff=777561371&oldid=777560645
I think that I may have reported this (without figuring out what the cause was) a few months ago, so there's probably a duplicate report running around somewhere.
Given that reference list templates are not fully supported, not widely used on most wikis, and that a specific and uncommon set of actions must be taken to cause this problem, this is not a high priority task.