ok so i managed to figure it out,
all the layers i am using are vectors and the little bubbles that were causing the headache were vectors in precomps (due to animation needs)
i hadnt turned on the “continuous rasterise” button on the lower layer and some of the motion blurs on the layers in the pre-comps werent turned on either.
Still dont see how this would affect what looks like a z-depth issue but ah well, hope my ramblings might help someone else out in future…