When dragging a macro which is listed as a subroutine, from the list of available macros on the left, and dropping it into the macro you're editing the step should be inserted as a "execute subroutine". Instead it's inserted as a "execute macro".
Probably it should create an Execute a Subroutine action, but a macro is still a macro - having a Subroutine trigger does’t mean you can't execute the macro, so its not wrong per se, just probably not the best thing it could do.
As Chris suggested, add a right click menu option to switch between the two types. This would help where already written macros are converted to subroutines.
Drag and dropped subroutine gets inserted as a subroutine.
Drag and dropping a subroutine asks which way to insert
An additional checkbox property could be added to the subroutine to emphatically state drag&drop as a version 10 subroutine.
Off topic: Being able to use return instanced variables is a huge leap forward. Thank you for making this a reality.
Dragging a subroutine triggered macro to an action list creates an Execute a Subroutine action. Dragging a macro link to a macro group creates a macro with an Execute a Subroutine action.