I took a closer look at the sources and found the bug. When the dialog is
shown, QSecLineEdit->grabKeyboard() is called, so all keyboard events are sent
to the lineedit. When you press the Menu key, the menu is created and shown by
the lineedit, but you can't close the menu again, because all key events are
still forwarded to the lineedit. When the lineedit receives the Esc key event
it will forward it to the dialog which will close itself and hide all
childrens (the menu). However the menu is still there because it's event loop
is still running, that blocks the dialog from returning to pinentry.

D37: 115_pinentry-qt.patch