Confused! ATEM not sending control signals to BMPCC

There was a time when I could connect my BMPCC to the ATEM and it would happily accept the ATEM's timecode and start recording when the ATEM record button was pressed. Not so any more and I am getting very confused even trying to isolate where the problem is, and suggestions would be appreciated. The short version is that I have swapped around cables, ports, and cameras and have not been able to consistently reproduce the problem. However 90% of the time it doesn't work.

Sending video from the BMPCC to the ATEM is not a problem.

I am thinking there are 5 potential sources for this problem.

Cables
Camera HDMI
Camera configuration
ATEM HDMI
ATEM Configuration

I am 99% certain it's not cables. I have swapped suspect cables from the BMPCC to the Studios and they have worked correctly. Likewise in reverse a known good cable has not made the BMPCC start working.

Swapping to different ports on the ATEM in some cases will get Timecode working, but not recording.

And to make it frustrating, every once in a while it does work and my current theory gets blown out of the water.

The kit:

ATEM Mini Extended Pro ISO
1 x BMPCC
2 x Studio Pro 4k
1 x Canon R8 (new addition primarily for photos, but will probably add in as a video input)

Installed in Road Case which includes a server, networking, audio etc.

ATEM Connections
HDMI Inputs

1 R8 tentatively
2 BMPCC
3 Studio
4 Studio
5 Server - monitor 2
6 Server - monitor 3
7 Media Laptop
8 Streaming Laptop
Mic 1 not used
Mic 2 audio feed from mixer

Outputs

HDMI 1 Multiview
HDMI 2 Broken
USB 1 spare/webcam
USB 2 SSD

Any thoughts?

Thanks,
Darryl