There's a big discrepancy between two displacement methods in how they treat mapping channels. If 2nd or higher mapping channel of an object is empty and there's a displacement map in the material, which uses that channel, then new displacement ignores that map and do not displace geometry (which seems logical), but the old displacement for some reason is copying mapping from the first channel and pastes it in the empty channel (which seems odd).
I can no longer reproduce artifacts, which is nice, but displaced geometry still has strange shading on shallow angles with new displacement. I wonder if it's a limitation of 2.5D displacement, or maybe things can be improved here?