Thank you friends. Let me go over some details that might help someone else in the future.
This started because a user told me that visiting www didn’t work, it gave a certificate error. When directly visiting their https link they provided in the reporting thread, I met the same. In the past when using www I had not met the same, but had instead been redirected to the root domain without issue. This led me to the conclusion that my installation after a recent migration was somehow broken and not acting according to default characteristics of a new Discourse installation.
So I installed a fresh copy of Discourse on a new domain to set out to prove that www worked by default when using the root domain. I went to the domain, then I edited the URL in my address bar and added “www.” to the front of it. It redirected to the root domain without issue, as I expected. Then, I thought to try one more thing. I manually typed “https://” before it. It then failed with the same certificate error.
So what may have led me to assuming that signing for www was default behavior in Discourse’s LE implementation may in fact have been my browser defaulting to port 80 while hiding the http/https portion of the URL from my view as I rewrote the URL in the address bar.
This is my best assessment of the situation at least.