It seems YouTube, as of late, will sometimes happily obliterate the quality of YouTube encodes if they were done at a resolution between 4k (2160p) and 8k (4320p).
What's the worst that could happen? This:
(incidentally, this was uploaded at 5k resolution, and it does not have that quality option, when previously uploaded videos actually showed that option)
It should be noted that the background scrolls
insanely fast upwards in this segment.
By the way, this will affect
all quality options (I always double-check my encodes at 720p60, like in this instance, but 4k will equally be affected)
If it was encoded at 4k res, it will be fine:
(note: this is with Lanczos downscale, though I assume straight point upscale is also fine)
As an aside, this has happened to me (to a lesser extent) when I did The Mysterious Murasame Castle:
(note: 2688p encode, quality options do not go beyond 4k (expected))
The published encode does not have these nasty artifacts below the tree.
If anyone still wants to upload a video that's above 4k res, then it should be
double-checked that it's fine on YouTube.
Until YouTube fixes the issue, it is strongly recommended to not upload anything between 4k res and 8k res, especially with the 5k option no longer available on new uploads, even if it was encoded at 5k res.