Sony FX 6 Quick Start Manual page 121

Cine EI: auto exposure and Cine EI
It is better to use manual exposure when shooting in Cine EI.
The camera's auto exposure does not take into account the EI rating, so
if the EI preset value is different from the Base Sensitivity ISO rating,
the auto exposure will be incorrect.
Shooting with the same EI value as the ISO, the auto exposure should
work correctly but keep in mind that fluctuations in exposure during
the shot can make "normalizing" the Slog 3 images more difficult.
In general, it is better to use Custom shooting mode when using auto
iris exposure.