Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce RTX 3080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.79
Broken: version: 2.92.0 Release Candidate, branch: master, commit date: 2021-02-19 06:58, hash: rB241273d36261
Worked: (newest version of Blender that worked as expected)
Short description of error
When you render a pass that is a single value (eg. Mist, Z, value AOV...) when you switch to in in the passes dropdown, the value is interpreted as alpha in the viewer.
The display channels dropdown updates to colour only but does not properly update the viewer. Re-selecting the option fixes it.
Exact steps for others to reproduce the error
In the default scene, enable the Mist pass.
Render the image and change the viewer to the mist pass and the cube will show as transparent.