The new 'max_stack_size' item attribute does not support the new stack sizes that can be assigned to items.

For example when running this command '/item replace entity @s container.0 with minecraft:apple[minecraft:max_stack_size=99] 99' the  command throws an error saying :

Integer must not be more than 64, found 99

..._size=99] 99<--[HERE]

This command fails even though the item i'm replacing has the 'max_stack_size' attribute increased above 99.

The same also happens with single stackable items or items with a lower 'max_stack_size' by default

For example when running this command '/item replace entity @s container.0 with minecraft:diamond_sword[minecraft:max_stack_size=16] 16'

minecraft:diamond_sword can only stack up to 1