I think this may be it, although I believe it applies to all lights. After trawling the ESPHome source, I think you may need to include gamma_correct: 1.0 in your light section.
As far as I can see, the default 2.8 value is being applied to your 50% as an exponent, giving 14% duty cycle. The relevant function is currently line 74 of this file. ESPHome reference is here.