I agree, but I don't have a good solution to this yet.
However there is a possible workaround:
Create the submenu button and style it as you would like to have your submenu styled. Make sure to set the "Use Same Style For Submenu Where Applicable" Add your submenu items etc. Then DISABLE the submenu.
Afterwards add a button that gets the predefined action "Open Specific Submenu In Floating Menu" assigned:
That action will also open hidden/disabled submenus.