That link leads to your certificate, not the intermediate certificate. The intermediate certificate is linked as part of the response header (last line):
curl -i https://acme-v01.api.letsencrypt.org/acme/cert/03b829db51a4af2f6e4fa29c4025bdd40e08
HTTP/1.1 200 OK
https://acme-v01.api.letsencrypt.org/acme/issuer-cert leads to the correct intermediate certificate:
openssl x509 -in issuer-cert -inform der -text -noout
Subject: C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
I took a quick look at the ACMESharp source, and couldn’t find any hard-coded intermediate certificates and there is some code that downloads the rel=“up” link leading to intermediate certificates, so it looks like their implementation is correct. It’s possible there’s some non-obvious bug somewhere, so I’d suggest creating an issue anyway.