Many times, for some reason, when using a brush the Layer Adjust will become active. Changing brushes or tools using the Wacom pen has no affect. However, if the mouse is used to reselect the brush you were originally using, then the brush will become active and the Layer Adjuster tool will vanish.
I am using a Mac with MacOS v10.13.6.
If you are using an old Wacom driver, please uninstall and reinstall the latest one.
Let me know how it goes.