Update, because this is interesting. It did do it again, but not until (I think) 10% this time. (Might have been 11%.) Once again all three gauges were amber - driver's screen, main infotainment screen and charging screen on the central screen. Also, when I started the charge and locked the car, the charging indicator on the driver's screen was again amber.
When I've had the random amber on the main infotainment screen I don't remember the driver's screen doing it as well. I do remember it being amber when I went down to 4% once before, but there was so little of the gauge left I don't know if it was on the driver's screen or not.
This isn't entirely random, at these very low SoC levels, although it's not totally consistent either. It does seem to be coming on as a warning, just not as high as you'd expect - assuming that there should be a red zone below it. I believe I have seen red on some videos of charging trials with the MG4. Has anyone here seen the indicator red, at any SoC?