A few things:
1.) Entities other than the player detect blocks they're in as an air block.
/execute as @e[type=item] if block ~ ~ ~ oak_sapling run say hi will never run. /execute as @e[type=item] if block ~ ~ ~ air run say hi will run even if the item is currently inside a sapling.

2.) Entities other than the player have unreliable offset block detection.
/execute as @e[type=item] if block ~ ~1 ~ air run ... doesn't always work when it should, and sometimes works when it shouldn't.

3.) Entity "if" check inside a function not run by the player does not work.
In a contraption I'm working on, *execute as @e[type=item,nbt={Item:

{id:"minecraft:apple",Count:1b}

,OnGround:1b}] if entity @e[type=area_effect_cloud,distance=0..3,nbt=

{Potion:"minecraft:regeneration"}

] run tag @s add 0x_appleSaplingItem* (couldn't monospace it) works when the function it's in is run by the player, but not when the function is run by a command block, even if the command block executes it as the player.