Should support backendDataProvider and if this is a subclass you need to ad

Directory ComboBoxMultiselect Add-on
1

Should support backendDataProvider and if this is a subclass you need to add this:
@Override
protected Class<?> getSelectionBaseClass() {
return this.getClass()
.getSuperclass().getSuperclass().getSuperclass();
}

2

The support is there, you just have to call setPageLength() manually, as it is only set automatically when using setItems() and not setDataProvider().