This bug let's water float, it won't flow away, not even sponges work on it. It isn't a raw visual glitch, as it still acts like water when entering it.
Video that shows how to reproduce: https://youtube.com/shorts/N7PxGKVjJw0 (file exceeded size limit)
This might also work with magma blocks instead of soul sand. The bug was found by me and the video was made by me.
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I was able to reproduce it multiple times in different worlds, without any global resources or resource packs activated. It was on 1.21.20, I'm not sure if it was fixed in 1.21.21 already
I also just managed to reproduce this in 1.21.21 through the same way. The reason why you couldn't reproduce it may be because you made the "tube" too big, AFAIK the sponge should be able to "absorb" all the water, otherwise it won't work
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