I just know you have to be careful with the lights you fit. I also have a quad light beast that had really good low beam until I found it was all four all the time. The inner high beam inserts had a structure at the back for replaceable H4 bulbs that earthed from one pin to the body via the metal base the bulb sat in and the spring retainer - drove me nuts the night before presenting for roadworthy inspection , swapped switch and all sorts of checking. I ended up insulating the spring retainer with heat shrink then installed quartz sealed beam high beam units that just had the pins set in the glass - insulating things.
I have also learnt that to add a relay to lights and horns on these old Subarus to not only just tap into the positive for the relay but to use the same original earth/ground/neg- side of the original wiring circuit to avoid problems.
I also found a low 1.4V coming from the high beam circuit when just on low beam in an EA82 L series when I hooked up driving lights like yours. I ended up putting two basic diodes in line (series) in the relays trigger wire from the high meam circuit to reduce the Volts to a non trigger Voltage , ntil I actually had high beam on. Diodes are said to absorb 0.7V, I found I needed to to get it down about 0.5V.
Very sensitive relay trigger.
Some also use diode protected relays to prevent any feedback, diode inside them.
Loks like your headlight is having its earth or power sapped by something...