LED BULBS going bad, bleeding to other circuits

M4 bulbs are not cheap so that probably is not the problem. I use the real cheap bulbs from eBay without issue. The fact that you have other lamps coming on with low output tells me there is likely a bad ground connection. Use a voltmeter at each problem fixture and check that against battery voltage, there should be very little difference. LEDs are sensitive to low voltage, even the regulated ones if way too low.