They only work with the rubber boot on and may not be as strong as the one you played with.
If there is a magnetic interaction with the holder, then I'll agree that Gossen should at least warn their customers.
The Fluke 185 relay is also affected by a strong neodymium magnet held to the back of the meter, but only if taken out of its boot.
The 8 pound magnet at the tip of my magnetic wand had no effect.
The Fluke 867B case is so big, only the LHC may affect it in any way.
https://www.amazon.com/Magnetic-Telescoping-Retriever-Telescopic-Retrieving/dp/B0175B3VRU
When you remove the magnet and power off the meter.What happens when you power it on ?
3DB
Without any input, I can change the auto ranging from mV to V and vice versa at will, by reversing the polarity of the magnet.
On restart, the auto ranging click back to the correct mV range.
With mains connected, the relay clicks back to the correct range too.
With mains connected, auto ranging can be changed with the magnet from 500V to 1000V, which then shows an erroneous OL.
I couldn't find a scenario where the meter would display a low voltage with mains connected.
I've been using very strong double stacked cylindrical neodymium magnets.
So the Fluke seems reasonably safe.
Morale of the story: Don't use a magnetic holder if your meter is equipped with relays. Doh!