It appears to be stretched twice as large as it should be, which is unexpected. Can be seen with the attached resource pack.
It'd be expected that, when next to glass, water would map the full 16x16 texture, rather than only a quarter as it does for the flowing texture.
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Can confirm in 21w05b.
Can confirm in 1.16.5.

In 21w10a

In 21w11a

I have figured out that because the side of water uses the flowing texture, which is 32x32 instead of 16x16, this is probably intended. Please resolve.
Not unless mojang also says it's intended; it looks really off, and texture resolution of a block shouldn't matter.

In 1.18.1
Still happens in 1.21.1, creating an animation for this texture is bothersome as I have to create a 32x32 canvas and only use the first quarter for a 16x16 texture, having the other three unused, resulting in my output file being 6 MB instead of 600 KB.
Can confirm.