I was testing some command block mechanics which included giving the player a slowness effect. After a few tests I decided to record it but then the slowness effect didn't slow me down at all. At first I thought there is a command that clears the effect but there isn't!
The time of the effect was counting down as usual, so it wasn't cleared and I didn't have a speed effect.
Still I was moving with default speed and the FOV didn't change even though I had slowness 5 (/effect @a slowness 5 4 true)
After rejoining the map, without closing Minecraft, the problem was gone.

I'm sorry that I can't give any more information about it but maybe it helps finding the issue.