Multiblock Tweaker: stable-backcompat-1.0.3
Ender IO: 5.1.55
Forge: 1.12.2-14.23.5.2847

The in world previews of multiblocks render above Ender IO fluid conduits when the fluid conduits are in separate Blockspaces. An image can be seen here:

In this image, the top right corner of the fluid conduit is seen to be behind the inworld preview. This happens for all blocks in the preview, not just the current erroring block. This also only happens with the fluid conduits, tested with Ender Fluid Conduits and Pressurized Fluid Conduits, and does not occur with item conduits or energy conduits. Not tested with redstone conduits as I forgot at the time I had my game open.

This is likely a bug on EIO's end, as depth test is enabled when rendering.

Upon further testing, it occurs with other translucent blocks as well.

Having looked into it, I have deemed that there is no feasible solution. The preview would have to be rendered after solid blocks, but before transparent ones, which seems to be impossible.