This an update to my previous reply, two days ago.

It occurs to me that what I had seen was a required, leading ampersand and an optional, trailing semicolon.

I suspect the lack of ampersands in the very long list of "HTML Living Standard: 13.5 Named character references" threw me. I see the Note there mentions "... some appear both with and without the trailing semicolon ...".

I always include both the ampersand and semicolon. The one exception is in the post above where I was trying to be clever and failed miserably. :-(

Again, thanks for pointing this out.