HDMI splitters do not generally pass the camera control information (HDMI CEC data). The reason for this is that the camera control information flows in the opposite direction from the video output signal being split.
With HDMI connections, there is a source device (which generates a video signal) and a sink device (which receives a video signal). So the video goes from source -> sink, but the CEC data (camera control commands in this case) goes in the reverse direction, from sink -> source.
When using an HDMI splitter you end up with multiple sink devices; with 1x2 signal splitter, there are two sink devices that are receiving the video signal that could be sending HDMI CEC commands back to the original source. With a 1x4 splitter, there are 4 sink devices. Which sink device commands should the HDMI splitter pass back to the source device? From sink device 1? Sink device 2? All of them? What if they conflict with each other?
Anyway, since this is difficult to resolve most HDMI splitters just punt on this issue, and don't pass these signals at all.
If you search the forum you will find other people with similar results with HDMI splitters and the Pocket Cinema Cameras 4K and 6K.
You could try to seek out an HDMI splitter that claims to support CEC and ARC, and if you do find one that works with the ATEM Mini, be sure to report back as I don't think anyone in the forum has found a model that works for this yet.