adding to what I said above, the left side is what was shown higher up by @C0ND1T10NR3D and I would consider that a bug that may be fixed at some point (it is inconsistent with how fluid flow works, where fluids would “not” pass through on the left side example).
the right side is a limitation of the voxel system since once light/fluid enters the block, it cant be stopped from exiting somewhere else in the same block and has to ignore internal block geometry that is sub-voxel, so the right side allows fluid flow, and even with the bug fixed, would continue to allow light flow too.