MC-262020 has returned in 1.20 Pre-Release 6,

From MC-262020:

I have noticed when snifflets fall off somewhere they try to jump to get back to where they were even though it was >1 block so they can't get back to where they came from. Doesn't seem to affect adult sniffers unless they grow up while they were already jumping

It also doesn't seem to matter if they fell far enough to take damage. It happened with a snifflet that only fell off a fence (1.5 blocks) and they still jumped.

It only seems to happen if they don't purposely fall. Like if you push them or water pushes them off an edge. If they purposely fall they won't jump because they meant to go downwards and know they can't get back up.

Steps to reproduce

1. Make the setup shown in the screenshot (or something like it)
2. Stand in the water right at the water level and switch to spectator mode. For some reason it no longer happens if you are in creative, presumably colliding with the sniffer or it detecting you causes it to stop pathfinding.
3. run /summon sniffer ~ ~ ~ {Age:-300s}