Hi there,
So it may be a simple as the Addressing of it and defining it in the right way? I use it as extra storage in lew of a SD card. Works fine. 
I did see a while back someone on the Nordic support forum do that exact thing , created a slot for the OTA download, then did some flash tricks to Run the update. It’s native on the Nrf53 & 54 series. Nrf52 I believe requires some extra hoop jumping but it does work. Take browse over there and try the search on there too. You get a lot of info on it as they do it Very, Very well.
HTH
GL 
PJ
Here is one of several good threads on the subject of the flash partitions to support DFU_OTA slots.