ATEM Mini Pro ISO - chrome key black not working

I'm trying to use the chroma key on the ATEM Mini Pro ISO with song words, white text on a black background coming in from a PC but it isn't working.

I'm projecting using a piece of software called proclaim and I have a main input that's going out to a projector, which has a background and looks nice etc.

Then I have a 'confidence monitor', which is coming out the ATEM, which I can reposition the text in, so it's at the bottom to go on a stream but frustratingly, I can't change the background colour on it.
And for some reason, when I try setting the upstream keyer to use a black chroma sample, it doesn't work and turning the key on brings in the text with a black background.

I've tried putting a green output on the main background and sending that into the ATEM instead and that works great - but I need the main output for the projectors in the building and the same steps I used for the green background don't work with the black one.

Change from Chroma key to luma key.

I tried that, and it works but you can't turn it on and off which isn't ideal.

For some things you have to use the software. I would suggest that on the computer that you have running Atem software to also install Companion software and connect a Elgato stream deck to it. Then you can have a button to mix it on and off again. Just set up a macro and assign that to a button.