System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: Quadro M2200/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.23

Blender Version
Broken: version: 2.90 (sub 0), branch: master, commit date: 2020-04-28 11:02, hash: rB8da80e7771fd
Worked: none - tested also on 2.82 and 2.83

Short description of error
If you start Blender with OCIO and you import an Image as a Background image for the Camera the colors and contrasts are not displayed properly.
In my Example I used the same Texture chart for Background and the texture on the Plane with the same transform. If you import the texture with an Alpha Over for the Output the colors look like expected.
After some testing it seems that Blender is applaying some Gamma Correction on the Background Image what it should not do.

Exact steps for others to reproduce the error
Import the same texture as Background Image and as plane. Attached is a simplified .blend that should be opened with the Standart ACES 1.2 OCIO. (OCIO 1.1 shows the same result)

Thanks for this great piece of software. If something is missing then please sorry. This is my first bug report. <3