I believe there is a bug in the Sertorius scenario regarding replacements. Any damage to Sertorious' legionary cohorts appears under the Roman tab for replacements. Although completely destroyed elements will be rebuilt from scratch when parked at a depot, partially damaged elements will not be repaired, as evidenced over the last several turns of a PBEM. This contrasts with casualties to the legatus legion elements, where damaged elements appear in the replacement pool under the Sertorious (not the Roman) tab. Those legatus elements repair normally, whether completely destroyed or just damaged. This leads me to suspect that somewhere in the replacement code, the Sertorius legionary cohorts are mis-identified, and probably need to be switched to the Sertorius tab like the legatus elements.
I can include turns from my PBEM game, but it's probably the kind of bug that can be identified by just popping into the scenario and running a quick test.