Hey Rob,
in its stock configuration, the Raspberry Pi 3 that is inside the Onefinity Controller is set to auto-detect the video mode. Of course it can achieve this only if the monitor is already powered up and ready, because it requests this information from the monitor. If the monitor is not ready, the Raspberry Pi boots into a very low default video resolution (as you already found out).
However, in all situations where you don’t connect a different monitor every day, this auto-detection is not necessary. In that case it is possible to enter the resolution of your monitor in the Raspberry Pi and it will always boot to the correct resolution, even if the monitor is not on yet. This is desirable if you don’t always want to switch on the monitor first, but at the same time as the CNC controller, e.g. with the same power strip. How to do this is described here.