I'm closing this as this behavior is by design.

GraphicsFormat is by-design a low-level format that needs to be queries in order to be used (using SystemInfo.IsFormatSupported).
Unless SystemInfo.IsFormatSupported reports that a specific format can be used for a specific use case (render/sample/…) on your specific setup (OS, GPU, Drivers, …), you cannot do it. The philosophy of GraphicsFormat is that if it is reported that you can use it, it can be used directly on the GPU (without any intervention from Unity).

This is opposite of TextureFormat. All TextureFormats should be able to use on any platform but without ANY guarantees about what exactly is happening in the background. For example compressed textures might become RAW (with additional decompression on the CPU), the GPU resource might be another type, … The philosophy of TextureFormat is that we (Unity) might jump through hoops in order to make it easy to use (at a protentional memory/performance cost)

This is exactly what your issue is highlighting. R8G8B8 is a format that (afaik) virtually no GPU actually supports. Some older APIs might emulate it in the driver and that is exactly how we thread it in Unity: We convert it on the fly to R8G8B8A8.
So when using the GraphicsFormat directly, an error is expected.

Best Regards,
Dieter