We run in to this all the time. Actually, we abandoned the CIVIL_ITL_File variable altogether for this very reason. I would suggest you simply open the ITL file each time you need to work with corridors. By browsing to and opening the ITL each time PW will manage it correctly. It is a horrible workflow and something that should have been fixed by Bentley LONG ago, but, it has not been changed to date in any way from initial releases.
Marc
Marc