https://bugs.winehq.org/show_bug.cgi?id=50169
--- Comment #9 from Czcibor Bohusz-Dobosz czbd@o2.pl --- TL;DR By 9faa5eeddd24a057d9ff522259c9dbdc6203c098, it's apparently fixed, thanks!
After finding that CFLAGS="-march=x86-64 -mtune=generic -s -O2 -pipe -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0" CROSSCFLAGS="-s -O2" CPPFLAGS="-D_FORTIFY_SOURCE=2" LDFLAGS="-Wl,-O1,--sort-common,--as-needed,-z,relro" consistently make Wine 5.21 builds that correctly run FSX, but still failing to bisect (the new attempt pointed to be8a81b54c7adf1e77d9411d337b9227e0eb119f, which, in two following separate manual builds from a clean state, came out working in one, broken in the other... I don't know what to think of this), I have just tried building the latest commit at this time (9faa5eeddd24a057d9ff522259c9dbdc6203c098), and FSX consistently loads with several rebuilds of that one.
So, yeah, the bug is fixed, it seems. And still is the weirdest software malfunction I've seen in a long time. As well as a good lesson for me not to put a status like "(bisected)" in the title of a bug report again... :)
Great thanks to everybody working on Wine, a difficult and truly important job.