Spot light through fog volume, better result using CPU only

My scene has a few spot lights shining into cone-shaped fog volumes. The spot lights do use an IES profile node. I’ve noticed a much better result on the fog volume rendering when in CPU only mode. Here are my settings, is something incompatible with GPU rendering?

Render settings:

PC specs:
Blender 2.8 Feb 20th release
Win 10 Pro 64bit
i7 3770k
1080 Ti

With the exception of CPU/GPU rendering no settings were changed between the images below:

CPU only:


GPU only:

Can you try this in 2.79? I remember having this problem several years back in an older version, perhaps 2.72 or something like that.
Back then it seemed to me that GPU had some sort of maximum samples and simply wouldn´t use more even if you put a higher number into the samples field.
A Blender version later the error was gone.

Some volume sampling methods are CPU only, see