[WP8548][mangOH] Open drain is not supported for GPIO output

I created a sample application for the WP8548 on my mangOH rev 8 board with firmware 16.10.3.
I try to control some LEDs (via an external pullup resistor) using the GPIO1-4 pins on the IOT0 connector.

Apparently there is no support for the open drain, tri state and high z functionality.
I already created an issue on the legato github (see github.com/legatoproject/legato-af/issues/16),
but, I would like to understand if there would be a reason on hardware or system level for this issue.

Does the WP8548 controller have any constraint on its GPIO output?

The forum for mangOH is shifted to forum.mangoh.io. Please post your query on that forum.

Thanks & Regards,