--rskip 2 coupled with --limit-tu 0 and --ctu 64 produces low quality

[SI]

I’ve recently done some tests with --rskip 2 and noticed that if it is coupled with --limit-tu 0 and --ctu 64, it produces a much lower quality output at the same CRF compared to --rskip 1 or if limit-tu is 4 or CTU is 32. The amount of 64x64 merges is much higher in the --limit-tu 0 --ctu 64 encode.

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My results are reported here:

https://forum.doom9.org/showthread.php?p=1919347#post1919347

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You can use this sample clip to test but it is reproducable with other samples as well, also when encoding at 1440p. With --rskip 2 --limit-tu 0 --ctu 64 --preset slower --aq-mode 1 --aq-strength 1.0 --crf 20, the encode of that sample shows quite bad artifacts in the flat areas around the middle of the clip with the woman on the phone in the distant background.

https://drive.google.com/file/d/1nEa9H70d7Ekeue2omnwLDToVK7LaxZ_t/view?usp=sharing

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