This is just more a problem solve if anyone else gets the issue.
I have a complex VFX project where certain areas needed to be retimed but others not, and running twixtor on it directly caused artifacts/warping, so I pre-comped roto'd areas for control.
Anyhow.. in all the ram previews it looked perfect. But when I rendered out my DPX sequence artifacts/warping were plaguing the edges of motion. I racked my brain for ages, because rendering frame stills individually was working but the comp was too long for that. BUT THEN! I went through all the layered comps and noticed the original footage layers had motion blur turned on for their layers but the composition motion blur 'on' button was not activated. So when I came to render as a sequence, AE activated the motion blur on those layers (even tho not displayed in RAM preview, intern causing the warping. All I had to do was switch off motion blur for each asset layer and its all sorted.
Hope this helps anyone with this issue down the line.