All lens or any other corrections are turned OFF. I have done more tests again. The issue gets better with the latest firmware update on the SONY and with lens corrections turned OFF, but it doesn't go away completely. Some color cast remains, particularly blueish color cast on the sides, where it's more pronounced in the corners. This affects both Batis and Loxia lenses.
I've shot dozens of time-lapses and now I have to do very extensive retouching in order to no have luminance jumps in my frames. Very frustrating and time-consuming affair.
if you own c1 use LLC corrections will save you a lot of time