Brain fart
OP here is how this works. "Let's say" you have the left front ungrounded. The parking or headlights are on, no turn signals. Because the fixture is ungrounded, the powered parking lamp filament has no direct ground, and power flows on through the TURN filament and that wire is connected to your turn indicator(s) which is/ are grounded, so that power lights up the indicator
When the turn signal is activated, the turn filament becomes powered of course during "flash" so at that point both filaments are powered with no ground, so the turn lamp won't light, but power is going to the indicator, which likely "lights brighter" during the flash.
Another great example was the popular Chevs about 60'ish to ?? 66?? which had "some" of the lighting in the TRUNK LID. Just think about THAT for a minute.........If the trunk lid had no good solid grounding jumpers from the lid to the body........and the wiring had problems.........and it often did........you used to have "interesting" effects........With the tail lights on, the lid mounted light would be dim and the body mounted lamp would be normal. when you activated the signal, the trunk mounted lamp would go out as the other one flashed turn.
The lid mounted light would get tail power.......no ground........through the tail filament........through the turn filament.......;and "maybe" to ground through the turn filament on the other side, through the brake light circuit in the turn signal switch.