Maybe the monster, or this witness, has the ability to freeze their victims in place?
My current theory is that nun girl somehow screwed up causing Tsubaki to be defeated. Then the other magical girl, the one that was with tsubaki, that i suspect have a some kind of relationship with nun girl, maybe they are sisters or something, messed up with Tsubaki's memories in order to protect nun girls and make Tsubaki believe she lost cause of her own mistakes.
I came to this theory for two reasons:
1) Tsubaki remembering being unable to react like she was restrained, that is the power of nun girl.
2) Tsubaki appearing very confused with many memory holes.
The problem is the dispatch magical girl refusing to deploy Tsubaki. She predicted Tsubaki would loose. Why? She can predict that other magical girls are gonna screw up or betray the deployed magical girl?