When it saw that for the first time (4/22), i thought i had it nailed down to the process of enabling developer mode on RokuTV. I.e. it happened after i had to re-enable developer mode after some FW change and after rebooting, suddenly there was not network and spontaneous reboots ensued. I tried different things and no one single item seemed to work - but at the end after some combination of driver reset, adapter reset, power unplug, factory reset - it seemed fixed. But then i was able to get it in that MAC=00:00:00:00:00:00 state again - if memory serves, through enable dev.mode reboot - and that time i somehow reset the adapter by lesser means factory reset and let it be, thinking i'll do a write-up on this later. Which i didn't.
Fast forward 10 weeks and the same thing happened yesterday, except i was not enabling dev.mode but simply rebooting the player (while researching the curious case of screensaver that has no name) when this happened. Same symptoms (all-0 MAC, spontaneous reboots) and driver reset, adapter reset, factory reset did not solve it but when i added to that removing the power cable, the network adapter was back.
Here are some screen captures i took back in April (then firmware was 6.2.3341, now shows 6.2.3486):
photos
I don't know if that's more of a hardware or software issue - but the part where it keeps bombing when the adapter is "not present" - that's clearly a software issue. It should be doing "the next best thing" and work as a "dumb" TV if networks are down, not crash. It shouldn't be happening, any of this. A casual viewer is unlikely to have my bloodhound tenacity in tracking issues, taking photos, writing down and so on - they'd just pack it up and return (bonus: bad reviews).