System Information
Operating system: Ubuntu 18.04
Graphics card: Nvidia 1080 Ti

Blender Version
Broken: Blender 2.80 (6d89337257b6 from December 25)

Short description of error
The cycles viewport won't stop udpating itself when a dupliverts parent object has a mirror modifier (or some other modifiers) applied to it. By "won't stop updating," I mean that it continuously renders sample 1 out of n. In my experience, this also stops the dupliverts from rendering in F12 cycles render.

Exact steps for others to reproduce the error

1. Create a plane
2. Enter edit mode on the plain
3. Select all vertices
4. Drag all vertices down, away from the origin
5. Add but do not apply a mirror modifier in the Z direction
6. Create a cube (as a separate object, so enter object mode first)
7. Make the cube a child object of the plane
8. Enable dupliverts on the plane
9. Enter cycles render mode
10. Enter rendered mode in viewport

Workaround

11. Apply the mirror modifier