Note: You do not need to respond to IOhannes directly as he gets messages from the list.
I can confirm this behavior, except for me the subpath window reappears on the right edge of the main screen. It does not appear offscreen. This is with both master and release 0.48 using Tk 8.5+. One issue might be TK 8.4 in Pd 0.47.
- try to re-open it (clicking object [pd subpatch] in the main patch) - the subpatch window gets opened, but doesn't appear in the display, it has moved outside screen