Operating system: Windows 10
Graphics card: NVIDIA GTX 1080
Broken: 2.80, 3d2ff33c2615, 2019-01-07
See attached .blend file and screenshot for a demonstration.
In 2.7 Shrinkwrap modifier with 'Keep Above Surface' allows to use positive as well as negative offsets.
In 2.8 positive offsets work fine but negative offsets are always interpreted as offset 0.