E1 and R3 have "parallel issuance" that depends upon the type of key (ECDSA or RSA, respectively) in the certificate signing request (CSR) for a leaf/end-entity certificate. Currently your ACME account must be on an allow list to receive ECDSA certificates signed by E1. If it's not on the allow list, your submitted CSR with an ECDSA key will result in a certificate signed by R3.
See @Nummer378's excellent explanation below for more information: