https://bugs.winehq.org/show_bug.cgi?id=44827
--- Comment #18 from wigglywoogly wigglywoogly@gmail.com --- So the solutions proposed by ajduck@outlook.com and Jan R didn't quite work for me, but they were a good starting point. I believe the reason they didn't work is that many of the 32-bit VSTs I was trying to modify only had a very minimal registry entry, and didn't contain the keyword "BridgedExternalWindow", and so attempting to modify these with a find+replace approach had no effect.
Instead by manually modifying the registry in wine regedit, I navigated to the entries for the relevant VSTs:
HK_USERS > S-1-5-21-0-0-0-1000 > Software > Image-Line > Shared > Plugins > Fruity Wrapper > Plugins
and found my VSTs listed by plugin author.
For each of the misbehaving ones, I created a new Hexidecimal DWORD called "BridgedExternalWindow" and gave it the value 1.
Then on attempting to run these VSTs in FL Studio, they open in External Window mode by default, and no more crashes. Happy days!
Many thanks to the other users who investigated this issue, and I hope this solution helps somebody else.