The Bug
If an entity (player, mob, armor stand...) falls on a bed/slime block while having the Slow Falling effect, it will bounce much less than normal, and that's OK, but when it finishes bouncing a weird shaking animation starts. This animation ends when the entity walks off the block or when the Slow Falling effect lasts. However, this issue will not occur when a player with Slow Falling falls on a bed/slime block while pressing Shift.
Steps to Reproduce
Place down a bed and give yourself the slow falling effect.
/effect give @s minecraft:slow_falling 100 0
Jump on top of the bed and watch your screen closely.
Take note as to whether or not players jitter when landing on beds with the slow falling effect.
Observed Behavior
Players jitter when landing on beds with the slow falling effect.
Expected Behavior
Players would not jitter when landing on beds with the slow falling effect.
Confirmed in 20w30a.