Never heard of any problems with Windows defender. However, is not very clear from your message if Couatl is being removed, with "removed" meaning physically removed from its folder, or just disappeared from the "Addons" menu.
If it's really removed, so you don't have the FSX\fsdreamteam\Couatl\Couatl.exe file anymore after running FSX, then it's SURE the antivirus has removed it.
If, instead, the file is there, but it doesn't start, it MIGHT be the antivirus (blocking it from starting, without removing it), but it might be also something else.