One Identity Manager 8.1 ADSAccount proxyAddresses

Why does AADUser (Azure User) have a built in field for proxyAddresses (and part of the default mapping) and ADSAccount does not?

To resolve this, I created a CCCproxyAddresses extension to ADSAccount and copied the design/structure of the Azure mapping in my AD user mapping in initial sync. I receive no errors during sync, however the proxy addresses are not populating in OneIM. The schema extension is of type Text, set to multi-value, and in every way I can see, identical to the proxyAddress field in AADUser. What might I be missing?