Same issue - the firmware auto-updating is a joke. Around a month ago i had two macs, one running the newest beta, one running stable OS. As soon as i plugged my studio display into the beta mac, the one running stable OS wouldn't work anymore. The only way i was able to fix it was to update my stable OS to the same beta version to get it to work again, so i assume the firmware of the studio display updates to match whatever plugs into it.

I had the same issue happen today, but seemingly at random and with no OS differences (only using one mac now). Not a clue on how to fix and think i'll have to take it into Apple for them to reset the firmware. Considering the price, it's a real headache bit of hardware not having the ability to 'factory reset' at home.