System Information
Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 419.35

Blender Version
Broken: version: 2.80 (sub 55), branch: blender2.7, commit date: 2019-04-09 18:12, hash: rB009dbc2bc9fb
Worked: (optional)

Short description of error
Workbench Engine - Camera Background Image disapears if the viewport display color of one material has alpha value <1.

Exact steps for others to reproduce the error
Open the attached *.blend.
Set the alpha value of the viewport display color <1.

The camera background image disapears.
The front option is working but you have to adjust the alpha value to see geometry.
The problem is only in workbench.