With the snapd solution, I'm fine (did the same as you, the chain1.pem to the same site, R3/ISRG1)
But, I just tried to add a new subdomain, then add the SSL with the LE plugin, and today, don't ask me why, not working
Maybe the DNS isn't yet fully replicated all over the world, so I'm waiting a little to try again.
Edit : I confirm, this was a DNS propagation issue (was a fresh new subdomain), just retried now (22:40) and it pass:
R3 by ISRG X1, so this is working.
I just got a little issue, the "cross-signed" certificate in CA-Certificate, I disabled those, wasn't able to use CURL with sites based on Let'sEncrypt SSL certs.