I think I found what might be causing this issue.
I was able to reproduce it consistently and found that runtime-created custom Scene Graph items stop displaying their custom name, description, icon and rarity correctly when they are granted to a player who is more than 15,000 cm away from the world origin. This distance appears to match the culling distance.
Within 15,000 cm, the exact same item and code work correctly. I also found a workaround by calling Drop[] on the item within a 15,000 cm radius of the player before adding it to the inventory.
I made a separate bug report with a reproduction video, steps and the workaround here: