I’ve noticed the same thing.
I believe that if you read the posts on the Yahoo webpage, it does indeed mangle the email addresses. If you use a built-in mail program (I use Apple’s built in on my MacBook and iPad), then the email addresses remain intact. So Yahoo doesn’t really strip it, it just displays it on its own webpage in a most unuseful form.
So, probably safest to do the “at” routine, even though it’s a PIA.
Aletes SM 240
St Augustine, Florida