ATEM Mini - Camera control lost when plugging in Raspberry P

I have a weird problem. At first I thought my ATEM Mini Extreme is broken, but it looks like it is something different.

I have 2 cameras. 1x BMPCC 4K, 1x Studio Camera 4K.
I use them with 1x ATEM Mini Extreme. (I borrowed a 2nd model for debugging, the behaviour is the same, so I think we can rule out broken hardware.)

All devices are on their newest firmware version.

Everything works fine, exactly as expected.

I also have a Raspberry Pi 400, which I use for overlays etc.
As long as I only connect the 2nd HDMI port to my ATEM everything is fine.

As soon as I connect the 1st HDMI port of the RaspPi to my ATEM
I loose Tally & control of both cameras.

I tried with different cables. Multiple different port combinations.

The behaviour is 100% consistent:
I connect RaspPi HDMI 0 to the ATEM, I loose the channel to the cameras. Data from the cameras is still working.

Any ideas would be welcome.

That’s problably a CEC problem. I have the same problem when I connect an older MacBook. You can solve this by putting a CEC-killer between HDMI cable and Atem input.

I ordered a CEC killer for further testing, but delivery is currently at ~3 weeks.

I did a bit more testing, and legacy fbdev seems to be part of the equation.
Loading vc4-kms-v3d fixes the issue, but complicates my workflow.

Still feels weird that a device on HDMI IN 8 can nuke functionality on HDMI 0 & 1.

I ordered a CEC killer for further testing, but delivery is currently at ~3 weeks.

I did a bit more testing, and legacy fbdev seems to be part of the equation.
Loading vc4-kms-v3d fixes the issue, but complicates my workflow.

Still feels weird that a device on HDMI IN 8 can nuke functionality on HDMI 0 & 1.

Is it working since the CEC killer? Got same problem with a Atem Streaming Bridge on Input 8

As you are using legacy, try if hdmi_ignore_cec=1 in config.txt helps.