This firmware upgrade killed the video signal on my slave remote.
When I did the upgrade, I didn't have my slave remote powered on, so it didn't get the RC firmware upgrade. At that point, the video worked. Then I upgraded that RC's firmware via the GO app; after that, everything worked (including live histogram) except video.
I spent an hour on the phone yesterday with DJI support. They said it was a known issue, but none of the things we tried fixed it. So they started the RMA process.
Today, I saw that there was a new version of the RC firmware available (1.5.70). I upgraded the RC to that, and the video signal came back.