The command syntax of /item is inconsistent with some other similar commands (especially /loot). The current order can also be problematic in the future if someone wants to expand it.
Current behavior
/item block ~ ~ ~ container.0 replace ...Expected behavior
/item replace block ~ ~ ~ container.0 ...Other commands to compare
/loot replace block ~ ~ ~ container.0 .../data modify block ~ ~ ~ ...
Can confirm in 20w51a.