When CSR was generated externally (like in Zimbra admin console, Openssl, Online generator, etc) and you use that during SSL order (Use existing CSR), Billmanager will complain that CSR has SAN and give error. Billmanager should ignore that and just proceed with order.
When ordering directly from Sectigo, they ignore the SAN in CSR. Why? Because at later step of order, you will be asked to specify the SAN(s) anyway.
When generating CSR internally within BIllmanager, pls add support for 4096-bit and ECC options. Currently, Billmanager only supports 2048-bit and RSA. Even Lets encrypt by default is now generated with 4096-bit!