There have been many reports on bugs like this, but none are exactly the same.
Whenever I ride an entity that is tamed and saddled, slow falling and jump boost don't work. However, if I'm currently not riding the entity the effects do work.
The tamed and saddled entities that this applies to are:
Horses
Donkeys
Mules
I tested this out on pigs and striders as well, but since they are not tamed the effects still work. I also tested it on a skeleton horse, but since it doesn't use a saddle (And you technically can't tame it) the effects also still work.
Linked issues
Comments
kiizde
GoldenHelmet
Special case of MCPE-45823.
osfanbuff63
@unknown That comment is from July, when 1.17.2 was the latest version.
migrated
Oh, my bad. I guess I got mixed up with the emails XD. I'll remove that comment.
Is this an ongoing issue in the latest version (1.17.2)? If so, can you please add it to the affected versions (or mention it if you are not the reporter).
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