Drop the _INTERNAL part. The documentation for the Python API is a bit terse, but at least for the “low level” functionality it is pretty much 1 - 1 with the C API, where the documentation for cdb_sub_abort_trans() (in the confd_lib_cdb(3) man page) says: Details can be found in the EXTENDED ERROR REPORTING section in the confd_lib_lib(3) manpage. - and that section says:
As CONFD_ERRCODE_APPLICATION, but the additional error information
is only for logging/debugging, and should not be reported by
I.e. it seems it is working as designed and documented.