https://bugs.winehq.org/show_bug.cgi?id=57135
--- Comment #2 from Rafał Mużyło galtgendo@o2.pl --- on a semi-random note: I've ran a few times (though it's extreme rare) into a case were a window gets created, but is invisible (either completely offscreen or for some reason without WS_VISIBLE). Might be the case here.
Checking for window presence can usually be done with relevant linux tools, checking the flag needs a Windows tool.