In 1.16.1 when you dropped an item onto a slime block it just landed and slid. It would only bounce if that slime block was then pushed by a piston. However after doing some experimentation in 20w28a I found that items bounce off of slime blocks as a mob or a player would.  According to MC-46257 the intended behavior of slime blocks when items are dropped on them is for them to just sit there so I think this is a bug.

To recreate:

1.Place a slime block

2.Drop any item on it