I've been reading that 2016-17 T120s had a poor reflector design for the high beams and people swapping them for the T100s. Did Triumph fix this problem in later T120s, such as the 2018 or 2019?
No, the bike still comes fitted with the LED 'smile' at the bottom of the lens. This reduces the area of the silvered reflector used to shine light out of the headlamp.