crouching doesn't prevent you from falling off hanging lanterns

That may be intentional as you can go down a half block (slab) while crouching.

How do you crouch on a hanging lantern?

maybe its on a fence and your on the edge of the lantern?

also on [19w39a],[19w39b],[19w40a]

This happens, because the lantern has a 2 pixels high smaller collision box on top. When you are standing on top of it, you can fall to the thinner part of the lantern, because the height difference is only 2 pixels, which is less than 5/8 blocks, so you can fall. And because you are not on the ground for a short time, you can completely fall of the lantern. If you make sure to always stand on the lower part, you won’t fall of the lantern as you can see in this video.

This is actually a duplicate of MC-2404, not just related; the lantern has 2 collision boxes, the one in the center is slightly higher (just like the model). Walking off, you're in the air for long enough to fall over the other box. Which is exactly what MC-2404 is saying, but with stairs/slabs as examples.

Can this still be reproduced in 20w27a now that MC-2404 has been fixed in this particular snapshot? If not, this will be resolved as duplicate of MC-2404 then.
Someone who was able to reproduce before please check.

confirmed in 20w27a

@pulpetti How can you reproduce this in 20w27a? I can not reproduce this, it seems like it is fixed.