Electronic & Hydraulic Shifting (Di2, Ui2, customisations)

I can lift the chain up to the middle of the cassette and then manually push the mech back, it feels like it has to go past a point then pops back into place.

This sounds exactly like the crash protection feature:
https://www.lfgss.com/comments/16091355/

When you move the cage manually it pushes the tooth round until it reengages with the notch on the motor.

Though it shouldn't come apart willy nilly, obviously.