I ran into a problem when I was first doing the Ultra96 hardware quickstart where I could not reliably connect my iPad to the Ultra96 WiFi access point. At that time I just bypassed it by configuring as a WiFi client.
Now I have that same problem when going through the PetaLinux labs and decided to look into it. It turns out that the 2018.3 BSP that I am using has the incorrect WPA driver specified. Apparently this is an issue that others have encountered before: Can't connect via wifi to Ultra96. Nor ethernet. Not sure why I only had a problem with the AP mode, but I verified that I had the wrong driver. The driver should be nl80211 instead of wext.
I edited the /etc/network/interfaces to make the change suggested in the post and now the AP mode works fine (client mode also still works). I'll need to go back and modify the BSP for future builds.