The annotation are not visible for the empty(placeholder) component because of below rules in css
http://localhost:4502/libs/cq/gui/components/authoring/editors/clientlibs/sites/page.min.css
.cq-Overlay.cq-Overlay--annotate.cq-Overlay--placeholder{
display:none;
}
Just add below rule in your custom cq.authoring.dialog css to see annotation for placeholder
.cq-Overlay.cq-Overlay--annotate.cq-Overlay--placeholder{
display:block;
}