Graph-on-parent is fine in PlugData. (Editorial comment: PlugData is a much-needed effort to move Pd's UI away from three-decades-old standards... assuming that it's wrong just because it's new IMO contributes to stasis, which is not healthy for the platform.)
The problem is in the structure of your patch.
sfplayer.pd is not a graph-on-parent patch. I can see this by opening it in regular Pd, and right clicking on the empty space --> Properties. The graph-on-parent property is not selected at the top level. So there is no reason for either Pd or PlugData to display it as graph-on-parent, when it isn't.
Inside that patch, you have a graph-on-parent subpatch. This is misleading you into thinking that you have done graph-on-parent, when you haven't. (But then, the GUIs are not in the subpatch either!)
Now, I do see an unfortunate limitation in PlugData: I don't see right away how to get to the graph-on-parent property. I expected right-click empty space --> Properties to show the canvas properties, but it doesn't. AFAICS then, to create a graph-on-parent abstraction, it looks like you will have to use normal Pd first.
Here is sfplayer with graph-on-parent defined at the top level.