This took a while to root out, but in v5.0.2 for Fabric 1.20.4 the Bar Panel won't render the block below it, acting like an x-ray, when closed if both Sodium and MoreCulling are present. This does not seem to happen when the Bar Panel is in the "opened" vertical position and only occurs in the "closed" horizontal position if there is a block under. This does not happen with the Lattice.

I tested this on Fabric Loader 0.16.2 for 1.20.4 with Decorative Blocks v5.0.2, Fabric API 0.97.1, Cloth Config 13.0.121, MoreCulling 0.24.0, and Sodium 0.5.8 with no other mods present. If either MoreCulling or Sodium is removed, the Bar Panel will render properly. I generated fresh, default configs to verify the issue during testing. After toggling different options, it seems that the BlockState Culling in MoreCulling is the cause or at least part of the issue and when disabled, the Bar Panel will render normally.

I am not sure if anything can be changed on your end to resolve this issue without disabling BlockState Culling, but since the Lattice does not have the same problem, it seemed like it might be something specific to the Bar Panel that could potentially be resolved. Thier GitHub wiki mentions "you can include the MoreCulling API and directly disable culling on your blocks". But there is also an option to disable culling by mod so that is a fix for the time being.