You can't place bells and lanterns on top of some non solid blocks

Not sure about the intention with bells and lanterns, but all flowers can't be placed on those blocks, and pretty sure that's intentional

Normally, Works As Intended.
It is the same question with hoppers (see MC-122352).

Fixed in 19w13a, with the exception of leaves, which is likely intended.

Yes it fixed in 19w13a.

Seems to be fixed.