Are you sure Eukrasian Prognosis is overwriting it and not Eukrasian Diagnosis? Because as Shibi mentioned, it's because Eukrasian Diagnosis shields broken through damage give Addersting stacks to Sages. They can't overwrite their own shield either (except through a bigger Eukrasian Diagnosis).

The whole shield situation is badly designed all the same, though, and just shows how incredibly pigeonholed healers in this game are.