Operating system: macOS 10.14.2
Radeon Pro 560 4096 MB
Intel HD Graphics 630 1536 MB
Broken: 2.80, 1b6b0fbd957f, blender2.8, 2019-01-08, built locally, no local changes
Worked: (optional): 2.79, 66c3eb369d274, blender2.29, 2019-01-04, 00:09)
Load the attached file
On the Object tab, enable and calculate the motion path - notice a smooth path
In the F-Curve modifiers (with the Z Channel Selected) add a Stepped Modifier, verify it is working in the graph view and in the 3D view.
On the Object tab, updated the motion path - it will become stepped
Remove the Stepped F-Curve Modifier - notice the curve in the graph window is smooth and the motion is smooth in 3D view.
On the Object tab, updated the motion path - it will stay stepped
On the Object tab, remove and re-calculate the motion path - it will stay stepped
Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).