When shooting s-log, if I set up a scene in stills mode so that the camera thinks the scene is correctly exposed, as soon as I begin recording the exposure readout changes from 0.0 to + 0.7. That is, it thinks it's over-exposed by 0.7 stops.
Another way to test the same thing, is to set up a scene in stills mode with the exposure meter reading 0.0., then without changing anything turn the dial so that the camera is in movie mode. The readout will change to +0.7. (It's usually + 0.7, sometimes more).
I'd be interested to know what's causing this.