Use Block Enroll to change grading basis?

Hello,

The Enrollment Mgr from one of our colleges at U of MN that uses Block Enroll is asking why she can't use it to change the grading basis from A-F to Audit?

In the Class Block, you can use the enroll action of Normal Maintenance and change the Grading Basis field from the default of A-F or S-N to AUD

by typing over the existing data, and tabbing out of the field but once the class block is saved, the Grading Option field reverts

back to the default of A-F or S-N.  I have checked that the course does have the AUD option at the Class Association level.