Yes, a forward looking org like Let's Encrypt would have said IPv4 if needed. Here is an example from Cloudflare https://[2606:4700:4700::1111]

Why does the url say one.one.one.one in my browser?

Because your are redirected to one.one.one.one via the location header and 301 status code from the ip address.

http://1.1.1.1 redirects to https://1.1.1.1 which then redirects to https://one.one.one.one

but the TLS cert on https://1.1.1.1 (or https://[2606:4700:4700::1111] on ipv6) is still valid for the ipaddress otherwise your browser would put up a warning during the tls handshake.

Its too bad it does the last redirect.

They didn’t used to. Guess they wanted to show off their shiny one.one domain.

Also (and just speculating here), it could be they wanted to get away from promoting https://1.1.1.1 because of legacy spam filtering. But that’s just me thinking out loud as to why they would prefer the domain over the ip

Because it returns a 301 moved permanently with a header of location: https://one.one.one.one/