The difference in bytecode is this:
csc
IL_0043: ldloc.0
IL_0044: unbox EnumParseBug.IndentStyle
IL_0049: ldind.i4
IL_004a: ldc.i4.2
---------------------------------------------------
mono
IL_0046: ldloc.0
IL_0047: castclass EnumParseBug.IndentStyle
IL_004c: ldc.i4.2