Not able to update the reference qualifier from 'Simple' to 'Advanced' for custom reference fields on a table. 

After changing the type and providing reference qualifier value, while clicked on save, the type is reverted to 'Simple'.

The issue may happen if the dictionary entry 'Reference qual' has 'Type' set to a value different than OOB.

In OOB instance, the 'Type' for this dictionary entry is set as 'String' and points to below URL:
/sys_glide_object.do?sys_id=7345e80cdb6c90d0c830bd5c689619d0

In the affected instance, if it is pointing to a non-existent sys_glide_object, then this issue occurs

Compare this field class with OOB:
Label-> User Roles (it is 'String' in OOB)
Name -> user_roles(it is string in OOB)

To resolve the issue, import the OOB field class through XML followed by a cache flush(cache.do)