Operating system: Windows 10 Version 10.0.18363 Build 18363
Graphics card: NVidia GTX 1070
Broken Blender Versions:
2.82 (sub 7), branch: master, commit date: 2020-03-12 05:06, hash: 375c7dc4caf4, type: Release
Also found on Blender 2.83 Beta
Lamp power is not updated in the viewport when changed using drivers. This only happens when using cycles in the rendered preview viewport shading mode. Eevee works properly.
Switch to render preview shading mode and change the custom property on the default cube. The property is linked to the lamp power, but the render preview doesn't change until you switch out of and back into rendered viewport shading mode.