System Information
Operating system: Win10
Graphics card: RTX2070S

Blender Version
Broken: 2.91.2
Worked: Untested, expecting never

Short description of error

Blender is able to render WebM files with VP9 encoder with RGBA channels. These videos work in Shotcut and Kdenlive video editors. However, they do not work as input in Blender materials, as opposed to for example Qt Animation

Screenshot of it working with QT Animation encoder:

Screenshot of it not working with VP9 encoder:

Exact steps for others to reproduce the error

1. Extract attached zip file
2. Open vidinblnd.blend
3. Set Viewport Shading to Rendered
4. Might have to do File/External Data/Find Missing Files
5. [Optional] Replace .webm with .mov in Shader input to compare