alter table wipes out systables.dirpath - SE5.0

I tried ALTER TABLE on the table whose database files are located in the directory other than $DBPATH/$DBASE_NAME. After table had been altered, I could not access that table. After some investigation, I found that dirpath column in systables for that table has been initialized. Has anyone stumbled on this one? We here use SE 5.0 on SCO Unix and AIX. I have called Informix support. Let's see what they say. I am preparing for workaround. Regards, Sadru Fidai.