Mobs never sneak and thus should always bounce on falling on slime blocks. However they sometimes collide severely and take fall damage.
I found it while testing MCPE-19451. You can test this bug with the same test world attached to it.
Linked issues
discovered while testing 1
Comments 3
Cleaning up old tickets: This ticket has not been updated recently (1 year+) so is being closed as Cannot Reproduce. If you feel this is still a valid issue, then please comment below and update the ticket description following the Issue Guidelines.
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It turned out that mobs only take fall damage when they hit an edge of slime blocks. This also affects players.