When spawning a FallingSand entity with block type minecraft:leaves2 and this entity drops (if it falls e.g. on a flower) it creates an item with data 4 or 5 (acacia leaves: 4, dark oak: 5) totally ignoring the original given data.
Given 0, 4, 8, 12: Drops 4
Given 1, 5, 9, 13: Drops 5
Other data values are not summonable, which is correct.
Since data values 4 & 5 don't have item textures they show as an unknown block texture.
The normal leaves block (minecraft:leaves) works fine and always drops items with data ids 0 to 3 as expected.

Reproduce:
Stand in a flower and do:

/summon minecraft:falling_block ~ ~1 ~ {Block:"minecraft:leaves2",Time:1}