https://bugs.winehq.org/show_bug.cgi?id=45369
--- Comment #14 from Zebediah Figura z.figura12@gmail.com --- Unfortunately this isn't really helpful as it is, partly due to the lack of any useful channels and partly because apparently kernel32 wipes the WINEDEBUG variable when launching the debugger anyway.
However, it does seem my initial guess was incorrect—the new explorer.exe instances seems to be launched by explorer itself and not by winedbg. But without any useful traces I can't determine why that happens.