It is not a bug! The solution was found. Our plugin was previously used for Illustrator CC 2018, so the new customizable toolbar introduced in Illustrator CC 2019 was unknown to us. In order to see all the tool-icons on the menu, all we had to do was clicking on "..." to open the "all tools" window, then clicking on the small settings-symbol in the upper right corner of the window and select "Advanced" instead of "Basic".
Previous problem description:
After compiling our plugin using the latest Illustrator-SDK, the tool-icons of the plugin do not appear on the menu anymore. In addition to that, they are not grouped together as they should be (sameGroupAs, sameToolsetAs). In order to find the icons, I have to click on "..." and scroll down to the Addons-Section.
I guess this is a bug because the same goes for the example-plugin "MultiArrowTool". When I compile this plugin and add it to the "Plug-ins" folder of Adobe Illustrator 2020, the plugin is loaded, but the MultiArrowTool-icons do also not appear on the menu and are only visible in the Addons-Section.