I was tech supervisor on a live stream this week. In our studio we had a simple setup with a presenter against a green background and a single mic. We joined a vMix Call session hosted by another studio in another country.
They asked us to provide the presenter with two monitors, one near to the camera and one off to the side for use while explaining graphics or interacting with an interviewee in a virtual set.
They sent us the picture for these monitors over the vMix Call return feed.

We had a test day, during which everything went well. Then, an hour before the full run-through on the day of the stream, we connect through vMix Call and then I find that I can't route their return feed through to the monitors. They're stuck on the input from our camera. No matter what I try, I can't get them to show the return feed.
I restart vMix, nothing. Restart the machine, nope.
It's a BM Decklink card; desktop video setup... Nothing to see there. I try different outputs but can't get them working.

Panic sets in. I have people turning the office upside down looking for monitors with DisplayPort so we can use Full Screen instead of External.
We're really eating into rehearsal time now and the client isn't happy. There's nothing we can do.

I try sending SRT and NDI to a laptop but WiFi is on a different subnet and the screen is broken.
It's a sweaty, angry nightmare.

Eventually the DisplayPort monitors arrive and the crisis is averted, though we've used up almost all the rehearsal time and don't get a break before going into a three hour stream.

After the stream I get a polite but firm debrief from the client, and while the crew are packing down I go through the settings again.

Then I finally figure it out: if I disable the external outputs, make my changes and then enable them again, it works.

Each time before that I had restarted vMix or the machine, enabled the external outputs and then tried to change the settings.

I don't know if this is a bug affecting only Decklink 8K Pro users, but if it's by design or if it affects other users can we please, please have a warning on the external output settings page:

CHANGES MAY NOT TAKE EFFECT UNTIL EXTERNAL OUTPUTS ARE RESTARTED

... because this would have saved me from the most stressful situation I've been in for a long time.