I attempted to set netherite backpack to 144 slots, but in world it shows only 12x10 backpack grid, upgrade slot count I set to 9, but shows only 6.

	#Netherite Backpack Settings
	[common.netheriteBackpack]
		#Number of inventory slots in the backpack
		#Range: 1 ~ 144
		inventorySlotCount = 144
		#Number of upgrade slots in the backpack
		#Range: 0 ~ 10
		upgradeSlotCount = 9

Results:

Using version 1.18.2-3.17.5.602 with forge version 1.18.2-40.1.52

Edit: actually, it's showing 7 upgrade slots. But the backpack is nested in the first slot... if you try to take it out, game crashes. This was after I opened my backpack, then opened a new one I spawned in. I also noticed in the picture my inventory was empty, except it was not.
When I opened it a second time, it showed 144 slots, but 2 backpacks in the top left. That was when I tried to grab one of those backpacks, and the game crashed.
crash-2022-06-26_23.51.38-client.txt

I also relogged in to my server, and... everything was working as intended, 144 slots, 9 upgrades, and no odd backpack dumped in the corner. So... odd.

have you opened that backpack at least once before you updated the slot counts?

that backpack was spawned in via JEI/NEI/REI whatever, after the config was updated.

closing old issues, this one likely was something related to curios desync I remember fighting with back in the day. anyway it doesn't seem to be happening anymore (haven't heard anything on that for at the very least 1 year)