Based on the mono mscorlib.dll source code, Environment.Is64BitOperatingSystem returns Environment.Is64BitProcess, which is wrong.
If you are running in WOW64 on windows, or the equivalent on linux/mac it should return true, not false.
What it should do, is if it is a x64 process then return true. If it is a x32 process, detect if it is a WOW64 process and return true if it is.
*** This bug has been marked as a duplicate of bug 37414 ***