The Flicker Free is for the UHD Cache, but from the short GIF, it looks to me like the flickering comes from the Bloom & Glare - looks like some pixels are being counted as within the threshold for some frames, but out of the threshold for others, so that the bloom and glare effects change from frame to frame.
Do you get any odd effects with bloom & glare turned off? (which is what the Flicker Free setting would control). Does adjusting the threshold value for bloom and glare make a difference? If you render a frame individually, does it give a different result than when rendered as part of the animation sequence? (e.g. find a frame where the bloom and glare is not showing in the current output, and re-render just that frame to see if it matches what comes out in the animation sequence).