Ritsuka guessed they'd both hit their heads, and while that could explain her own case, it couldn’t explain Medb’s, a Servant’s. It’s hard to believe a hit to the head could disturb a Saint Graph enough to make her lose her memories. She didn't doubt the hypothesis that she and Ritsuka were hit together, but she had no doubts there was some kind of magical interference involved.
(That’s probably related to the cause of the Singularity. The difference between my memory loss and Ritsuka’s is due to our different levels of resistance to magecraft.)

- Chaldea Case Files: Caged Girls