I also encountered this problem a couple of weeks ago, suddenly acme.sh was trying to renew one of my domains using ZeroSSL, when in all my settings I explicitly had Letsencrypt as CA.
I managed to fix the problem by registering and account with ZeroSSL, it's just a command which registers an account bu using an email, as it's explained here https://stackoverflow.com/questions/68538044/why-cant-write-certificate-crt-with-acme
acme.sh --register-account -m yyyy@yahoo.com
Once I did that then I was be able to create a new certificate with ZeroSSL, then because I didn't want to change Letsencrypt I forced a new certificate renewal by specifying
/root/.acme.sh/acme.sh --home "/root/.acme.sh" --renew-all --debug 2 --log --server letsencrypt --force
I still have no idea why acme.sh was trying to use ZeroSSL to issue new certificates, but it's been working fine since then.