Individual toolbox layout: Isocurve loses icon when copied

Kind of silly bug: If I copy the ExtractIsoCurve-button to my individual toolbar it shows up without icon. Saving the Icon and reloading it to the copied button works as workaround. Other buttons work just fine (just tested some).
Any ideas?
Thank you in advance,
Norbert.

Hi Norbert- what is the isocurve button? what command does it run?
thanks,

-Pascal

Its the ExtractIsoCurve Command. I added a picture above.
Best,
Norbert.