When a player spectates another entity, their rotation is rounded to a multiple of 1.4 degrees. Slow rotation therefore isn't smooth and occurs in steps.

How to recreate

/summon magma_cube ~ ~ ~ {NoAI:1b}

Spectate the magma cube. Open F3 to see your rotation. It should be 0.0 / 0.0.

/tp @e[type=magma_cube,distance=..2] ~ ~ ~ 1 0

The view doesn't change.

/tp @e[type=magma_cube,distance=..2] ~ ~ ~ 2 0

The view rotates, F3 reads 1.4 / 0.0.