I also use the Compilation Tag and have always found it to work correctly.
When you added that tag into MC did you set it's Type to Integer, in addition did you Add that tag before or after you Imported those tracks?
If the Tag is correctly defined in MC then selecting all the tracks where the Compilation Tag/Field is empty in the MC Database and use the Tools > Library Tools > Update Library (from tags) command will copy the Compilation Tag values, from the files, into the MC Database.