Did you had Defog active? I have seen flickers like this in one of my footage with Defog enabled and Antiflicker set at 50Hz.
After both were turned off, (WB always in Manual mode) no problems.
Seems that in some scenarios the Defog and Antiflicker start to...flicker

No idea why, possible that camera receives a fake flickering on 50Hz with Defog and tries to compensate it.
I have solved it by duplicating the clip another 2 times in Adobe P., base clip at 100% opacity, second duplicated clip delayed with +1 frame from base clip, put over base clip with Opacity 66%, third clip put over second clip and delayed +1 frame (so will be +2 frames over base clip) at opacity 33%.
This should help.
Also be careful...delay is on FRAMES, not seconds, as if you delay it on seconds, will be a mess.
Cheers.