if you disable FOV can be altered by gameplay in settings, the fov slider does nothing apart from staying in the same position. from what the toggle suggests, it should stop sprinting, speed, slowness, and similar things from changing the fov, but it seems to lock it at an extremely tiny bit over 63.60° instead of at the correct fov value.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-32403.
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