What could cause the Requirement Term to revert to a previous value with a Plan Change Action?

A user has reported examples of a problem with the Student Program/Plan action of Plan Change.  She submits a Plan Change where the Admit Term stays the same but the Requirement Term is updated to reflect a new term on the Student Program tab (one other than the stored Admit Term).  The Requirement Term on the Student Plan tab reads the change to recognize the new value. She successfully saves these changes.

However, when she revisits that Student Program/Plan the next day, the Requirement Term on the Student Program tab has reverted back to its original value (now matches the Admit Term).  The Requirement Term on the Student Plan tab remains the new value from the day before.