Leaf litter can be placed on walls and fences

Likely intentional, the 1.21.70.22 Bedrock changelog explicitly states that leaf litter can be placed on all blocks that provide center support (i.e. fences, walls, chains, end rods, etc):

Leaf Litter can now be placed on any block which provides center support

By "any block which provides center support" could likely refer to any block that has a full face on top. It doesn't seem intentional that they can be placed on top of any non-full surface.

"Center support" specifically refers to blocks that have solid 2x2 central pixels on the top face (this is also how placement for blocks like torches or lanterns works). I agree it looks weird, but this is how it's defined in code.

I suspect this decision was made in haste when they added central support for cactus flowers, hopefully they will revisit it because this definitely looks strange.

If we can put leaves, we should also be able to put wild flowers and cherry blossoms on fences, etc