It's not poor grounding - these are presumably LED turns I guess?
The reason all 4 come on is because of the OEM indicator lamp - this has much lower resistance relative to the now high resistance LEDs - so appears like a virtual 'short' path to the opposite side.
(The most simple test is to merely unplug your instrument connector plug or pull the lamp - if the turns then work normally, you have your confirmation)
The way the 'normal' idiot light works is:
The main current flows through the incandescent turn signals (Whichever side is selected by switch) to ground;
it also flows to a parallel circuit comprising (the indicator lamp (incandescent) in series with the opposite-side turn signals (incandescent)) to ground
The indicator lamp 'lights' because its resistance is much higher relatively (smaller wattage bulb) than the main turn signal bulb;
so proportionately more voltage is dropped across that indicator lamp than the 'opposite' main signal lamp (actually front & rear lamps in parallel) in series with it ;
so the indicator lights and the main lamp doesn't.
Now - when you install LEDs, they have very high resistance compared to the indicator lamp - so now the applied voltage is dropped over the LED turns as opposed to the indicator
- so both sides flash!
i.e. the indicator is now flashing on the opposite side because the higher voltage component in the opposite path is dropped across the LEDs, rather than the indicator bulb.
Hence the requirement to isolate that potential parallel path with diodes.
This is simplified diagram of how original indicator lamp functioned:
The solution is to do the diode mod for the indicator lamp
(note that on the Twins the negative for the high beam lamp is actually Brown I believe)