Currently no aggregated interfaces are discovered with net.if.discovery on IBM AIX and Oracle Solaris systems.
The function is implemented using perfstat_netinterface():
https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/performancetools/idprftools_perfstat_int_netint.html
And it simply does not support aggregated interfaces.

A request has now been sent to IBM about the availability of alternative methods to get this information

Depending on the answer, we expect either the corresponding functionality in the next upgrade =) or add a note to the documentation on the current state of affairs.