"Unable to modify player data"

Is this a bug? I have a command that should work to directly modify the inventory, to make it so my single player spectator server world makes it so everyone can see what's in my inventory.

execute as @a[name=!Roadhog360] run data modify entity @s Inventory set from entity @a[name=Roadhog360,limit=1] Inventory

However, it doesn't work, and targeting the player directly results in "Unable to modify player data". Considering this has a dedicated message I can't be sure if this is a bug.

But if this gets closed with 'Works as Intended'... Why? I don't see any reason to restrict this and it throws so many good redstone ideas right into the trash.