If I may take an educated guess:
Your system boots without waiting for the WiFi firmware loading to be completed.
This means that you have to unload and reload it by hand
('sudo modprobe -r iwlwifi' and 'sudo modprobe iwlwifi')
From another thread, the same seems to be happening to GPU driver firmware. (resulting in black screen)
The remedy here was to switch from arctica-greeter to slick-greeter because a bug in arctica-greeter prevents the boot to wait for loading of the firmware.
Something that seems to be happening here too.
(but I might be totally mistaken)
So, it might be far fetched and even not remedy this issue but you might want to try to switch arctica-greeter to slick-greeter to let the desktop wait for the complete loading of the WiFi firmware. It's not an intrusive change and easy to revert.