What version of IFS are you using? From App10 onwards, you could change the validity of the Purchase Groups through RMB and hide / block
In APPS 9 also you can remove the purchase group record unless it's not using any other reference records.
ex: Purchase part, Supplier discount, Purchase group supplier, Purchase Requis Line NoPart.
Technically, before removing the purchase group, you need to remove that reference first.
In your scenario its 'purchase order change lines'
To investigate why it's not showing purchase order change lines, it needs to technically investigate the
issue by checking the records in the database.
So Could you please create a support ticket for IFS support?
Then IFS Support can investigate the issue further and provide a proper answer\fix
Probable reason for why it's not showing purchase order change lines is user allowed site filtering. User can only see data related to sites that he has access, But there can be actual records in the database entered by other uses with the mentioned purchase Group.