The spec says:
caaIdentities (optional, array of string): Each string MUST be a
lowercase hostname which the ACME server recognizes as referring
to itself for the purposes of CAA record validation as defined in
[RFC6844]. This allows clients to determine the correct issuer
domain name to use when configuring CAA records.
The CAA identities are hardcoded to a dummy value:
Instead, the CAA identities should be taken from the corresponding CAA validator, or (perhaps easier) be specified as a setting per ACME alias.