System Information:
CPU: Ryzen 5 2600
GPU: RX 570 4GB
OS: Windows 10 Home

Blender Version:
Broken: Blender 2.8 Beta 925380050d0

Short description of error:
Adjusting textures used in a modifier will not show up in the Blender viewport until a modifier is adjusted or object mode is changed (E.G. Switch to edit mode then back to object mode).

Exact steps for others to reproduce the error:
Here's an example:

1. Open Blender and create a plane
2. Add a "Subdivision Modifier" with the view set to 6 divisions.
3. Add a displacement modifier and give it a new texture.
4. Modifer the texture used in the displacement modifier and give it something like a "procedural cloud" texture.
5. This will not be reflected in the viewport until one of the modifiers are adjusted or the user changes viewport mode (E.G. Enter edit mode then switch back to object mode).