I haven’t delved into the code, but as a user I’ve noticed various lack-of-full URL issues.
In my most recent Meta Digest mailing, I see full URLs in markdown format in the plain text part of the mail:
[:success:](https://d11a6trkgmumsb.cloudfront.net/original/3X/b/1/b1be...)
[image](https://d11a6trkgmumsb.cloudfront.net/optimized/3X/5/a/5af8...)
But I have seen ones like your example above (<img src="//discourse-meta.s3...) in the past. It may have to do with how the message was originally encoded, eg originally just a bare URL on a line versus in markdown.
The “Download My Posts” output uses the protocol less version here at Meta, but I’ve also seen lacking protocol and hostname (<img src="/uploads/default/original/...).