Probably because these items are marked as unique and only spawn at one location, the trader or the chest. The first one that you interact with will have the items (summoning the Tutorial Chest counts as interaction). I say that based on another mod that I'm testing.
For anyone playing with a mod list full of new magic items, and struggling to get the game to feel balanced, Zerd's RAW has an option to add attunement to BG3. You can match the tabletop limit of 3 items, or follow the Listonomicon example of a 5-item limit, or any more or less you think you need. To save you time unpacking every mod on Nexus so you can add modded items to the attunement list, I have made a file for this mod (and others) you can download/use/copy/modify/whatever.
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Check both places to be sure.