https://bugs.winehq.org/show_bug.cgi?id=44827
--- Comment #4 from ajduck@outlook.com --- I've discovered something about this behaviour that could further pinpoint what the underlying issue is:
It only happens *when you try to load the GUI* of a bit-bridged plugin.
If you load 32-bit only plugins (as in plugins with no 64-bit equivalent) to a project file in 32-bit FL Studio, close all the plugin GUIs and save the project, and then load the same project file in 64-bit FL Studio, FL does not hang when it's finished loading the project. The plugins work totally fine sound-wise, no hanging from that either.
It only hangs with the RtlpWaitForCriticalSection timing out when you try to open the GUI of one of those plugins. Vice versa for 64-bit plugins in 32-bit FL Studio.
I don't think it's to do with the rendering of the GUI itself, rather I expect it's something more to do with the process of loading the GUI, but this is a guess.