Was setting up a new virtual host in Apache on my RPi, which has involved much gnashing of teeth and questioning of life choices.
Went down a rabbit hole of HSTS preloading of my principal host causing the other to flip to it along with many other increasingly improbable things.
It was obviously DNS. My PC and phone (and router) were storing old values. Restarted my phone, connected to a different network and it was fine to install the Let's Encrypt certificate.
