I believe these issues with DC are related to the strict hsync/vsync alignment requirement of the digitizer chip. Alignment might be different between various VGA modules (if sync signals are not directly passed thru). Secondly, v1.5 and older OSSC PCBs have different R12 resistor (220ohm on v1.6, 330ohm on others) which also affects the alignment. Most certain fix is to use a sync combiner circuit and feed the signal to AV1, but you could also change R12 (e.g. back to 330ohm if you have v1.6 board) if you want to keep using VGA.