Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 419.17
Broken: version: 2.80 (sub 57), branch: master, commit date: 2019-04-14 18:55, hash: rB3731729b96fa
As well as older 2.80, 2.79b etc. This is not a recently introduced issue.
When you switch to a new Workspace that has the Material Preview visible, it will not refresh if the material has been changed while away from that Workspace.
Open default scene. In Properties, switch to Material tab and expand the Preview section.
Switch to Shading Workspace and do the same thing.
Change the Base Color of the object.
Switch back to the Layout Workspace and note the preview does not update.
AFAIK, it has always been like this but before Workspaces it wasn't as noticeable.
In other cases the preview refresh is extremely aggressive, for example it fires even if you just click on a node in the Shader Editor, start to drag a node, etc. Might improve general system performance if it only got triggered when something actually changes, but a simple solution to the not-refreshing problem would be to just always trigger the refresh when you switch to a workspace where at least one preview is visible.