The error usually appears as "ERR_SSL_PROTOCOL_ERROR" or "SSL_ERROR_RX_RECORD_TOO_LONG". This is occurring regardless of what browser is used and appears to be exclusive to Comcast Xfinity users.
My site is *.is-best-girl.fyi with the wildcard being used for obvious purposes, i.e. https://samus.is-best-girl.fyi/ .
It's on a shared hosting package with Namecheap, the computer in question running Cloud Linux, and they do not support root access or Let's Encrypt as an official cert provider so I had to manually install it with certbot running on Windows 10.
I contacted Namecheap customer support about the issue and they said that as far as they can tell the cert is installed correctly and the server is functioning properly.