Maybe the break switch (on the master cylinder) is tied into those systems, and when the bulb is on without the switch being pressed, it puts a voltage on those systems which they don't expect?
Otherwise, I'd guess it was programmed into the software as a safety feature to keep people from driving with their brake lights effectively disabled (because they only have one intensity, and never turn off?)
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View my fuel log 2017 Mirage ES 1.2 automatic: 42.3 mpg (US) ... 18.0 km/L ... 5.6 L/100 km ... 50.8 mpg (Imp)