Main menu screen background does not have transparent gray overlay anymore

Do you mean the beta cubemap?

Sorry I am not sure what a cube map is, but here is an image of what I am referring to. The left image has the overlay on the background image but the right one does not, which makes the background image look a lot more vibrant.

that's right I just reported the same problem ..... that gray background or fog is panorama_overlay.png apparently it is not working. I start the problem from the village an pillage version and it remains until now ... I hope that they are made to be too ugly without the fog (panorama overlay.png). thanks for reporting this you also friends

it is not the cubemap ..... the cubemap is in environment and this problem is in "ui" neither are the six panoramas ... if not the _overlay panorama ...

I show them with an image.
in the image the part marked with blue is the one that works together with the one marked with red but without a doubt the one with red marking is the one with the bug and does not have functionality. please take this into account and take it seriously

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However, this issue has been temporarily closed as Awaiting Response

Is this still an issue in the latest version?

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Still an issue in 1.16.210.53 Beta.

As you mentioned in Report is correct in  cases, but there's a guy named CrisXolt made a pack called "RGB GUI", in that Pack Panorama Overlay is working. 

But that's the only pack I saw with working overlay.

A screen shot of his pack -

Adding this file in the ui folder in any resource pack will enable the panorama overlay.
What it does is to add in the ui a linkn to the overlay texture and add it to the controls so it does render in the main screen

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However, this issue has been temporarily closed as Awaiting Response.

Could you please attach a functional resource pack showcasing this issue?

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This is not a resource pack issue. Just open the game and look at the panorama.