When swimming in any body of water with the sprint key held down while swimming, the FOV frequently switches between sprinting and not sprinting. This issue seemed to have been resolved in a 1.13 pre release, and possibly even a previous 1.14 snapshot, but has since returned. (See here)
MC-148221
FOV when Sprinting in water (Returned Glitch)
Resolved
Duplicate
0
Zachary
(Unassigned)
Unconfirmed
Minecraft 1.14 Pre-Release 2
Retrieved
Caused by MC-133414. The player can't sprint in shallow water (the bug), but starts sprinting once it is high enough in the water.