A7R V – How to fix Unexpected shadow noise in 4K 50p S-Log3

My camera is Sony A7R V - Filming interviews and B rolls
I usually shoot my B-roll in 4k full-frame, but since I recently started using the oversampled 4K Super35 mode for interviews, I decided to try shooting my B-roll in the same cropped mode as well.

so I did 4K 50fps, and as expected the camera automatically switched to APS-C / Super35 (I know this is normal at 50/60p).
My settings were: Shutter 1/100 / S-Log3 / S-Gamut3.Cine / ISO 800 (a7rv base) / Exposure looked fine on set

But I noticed a lot of grain (noise) in the dark areas, even though the image doesn’t look underexposed.

Is this normal when shooting S-Log3 in Super35 on the A7R V?
Or did I miss something with exposure ?

Screenshot of the noise is attached

How can I fix those footages ?

and Any advice on how to avoid this in future shoots would be appreciated.
Thanks!

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