When in spectator mode, go in water, and everything would be normal.
However, when you go in a block, the screen shrinks a bit.
Not precicely visible from these screenshots, but you can see the difference in game.
Tested at 1.16.4, 1.16.3 too, other versions could be affected possibly.
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Comments 4

The fix in 21w10a doesn't work.
FOV and speed still change
Actually duplicates MC-214630

Not actually fixed yet in 21w10a. Please be patient while mojang is looking into this issue
FOV changes when underwater, then fixes itself when flying into a block. Fixed in future snapshot.