So You are running Minecraft in fullscreen / windowed.
you tab out game/minimize it, Then tab in game again & you'll have Black screen IF ray tracing is turned on. meaning that it only affects ray tracing.
this is a critical issue as tabbing out/in game is very common, I currently don't have acess to an evidence image, but It's black screen with UI there only, always reproducible & takes a few seconds to do.
hope I explained everything.
UPDATED INFO: Engine switches to DX11 after minimizing and reopening the game, causing DXR to stop working
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This issue only occurs after Ray Tracing has been turned on in a given game instance; minimizing and reopening the game before ever turning it on and turning it on again will result in it working, but minimizing and reopening again after that results in the same issue.

For some reason the rendering engine switches to DX11 after minimizing the game, which is what causes the black screen and ray tracing to stop functioning.

Here is the link to the resource pack I used to take screenshots of the issue, but it does affect all packs as regardless of using a ray tracing compatible one the engine still switches to DX11.
https://www.mediafire.com/file/r4iatv86doxtxfo/Defined_PBR_v1.0.4.mcpack/file
Steps to reproduce:
Open the game and open a world
Minimize the game and reopen it (at this point there should be a black screen if ray tracing was ever turned on in the world)
Close the world and reopen it without closing the game
Observe as the rendering engine changes from D3D12 to D3D11, which both lowers performance and breaks DXR

Another bug report here (MCPE-104207) cites the exact same issue.

Affects release 1.16.210