If a part is transparent, SurfaceGUIs always render on top regardless of the decal being in front of it
Example:
Surface GUI Grand Clock
Surface GUI Grand Clock with random decal in front of it
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It’s a known bug; maxvee is working on it last I heard.
Bumping just in case 
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I have found numerous bugs with decals rendering. I’ll post a thread about it later. And yes I have seen this too:
It would be nice if it didn’t render through decals. I am fine with it going through transparent parts. Why? Because I use SurfaceGuis for a legit working TV and I’ve to tell you it is really realistic from the outside. Basically it’s how a lighted screen is shown.
SurfaceGUIs are GUIs, they are rendered in a separate pass and are not depth-sorted with other transparent objects. We’re probably going to do something about it some day.
It’s possible you could make them render an image version of what is in them, like a Decal. But then user interaction wouldn’t work.