Hey, I have exactly the same issue.
It happens when the MBP wakes-up after sleep (some hours) using an external display.
I think it's software related, I made a NVRAM & SMC reset, reinstalled everything and it's the same. Some other users exchanged they MBP for a new one and the issue is showing after few days too.
I found a workaround for this, the idea is basically reduce the pressure on the NVRAM. I think due a firmware/GPU driver issue there is a memory leak.
The idea is under Energy Saver disable the Power Nap, that will reduce some pressure on the NVRAM.
This hide (the but is still there) the bug, but at least is a way to keep working up to they release the solution.
BTW, this is my first post at the forum
