Hi, I hope I didn't overlooked an already existing bugreport but pretty sure it isn't WAI.

Basically you can add items to an Inventory of a piglin but can't remove it or set the count lower via /data. But it works for things like Handitems of the same piglin. It even duplicates every (non selected) slot of the piglin inventory.

Steps to Reproduce:
1.) spawn a piglin via

/summon minecraft:piglin ~ ~ ~ {IsImmuneToZombification:1b,NoAI:1b,Inventory[{id:"minecraft:stone",Count:2b}]}

2.) try to remove the stone:

/data remove entity @e[type=minecraft:piglin,limit=1,sort=nearest] Inventory[0]

It says "Modified entity data of Piglin"

3.) run

/data get entity @e[type=minecraft:piglin,limit=1,sort=nearest] Inventory

 We get the feedback "Piglin has the following entity data: [\{id: "minecraft:stone",Count: 2b}]"

Please note:

• Works too, if the piglin itself picket up items (will pick up if in the piglins loved tag).
• NoAI:1b was set so the piglin don't wander of and IsImmuneToZombification to work in the overworld but these aren't necessary at all to see the bug.
• If you try to override the Inventory list with another one, like with chest placed right under you

/data modify entity @e[type=minecraft:piglin,limit=1,sort=nearest] Inventory set from block ~ ~ ~ Items

it will add all the items of the chest   but will not remove any from the piglins inventory.

• If you do

/data modify entity @e[type=minecraft:piglin,limit=1,sort=nearest] Inventory[0].Count set value 1

 it will add all positive values but not substract negative ones. 

• Yes I'm on a fresh singleplayer world without datapacks and it happens without any mods.