With the mkdir() I believe the authority will default to create the object with the parent dirctoty authority.
One exception is if the object being created uses the CPYTOIMPF as IBM has told me that this command when creating a file from the AS/400 database to an IFS object does not support adopting the parent directory authority.
We have found that out with JDE EnterpriseOne created objects to the IFS for transfer using the CPYTOIMPF command.
We have to modify the program or run a script to place authority on any items in that directory and run this several times a day or incorporate it into the scheduler job to run after the job that just created the IFS object.
Hope this helps