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.
Comments 3
This seems to be caused by byte accuracy. The rotation increments client side in steps of 1.4 degrees. 1/256*360 = 1.40625
Can confirm that this is an issue. Also affects 20w51a.