Edit : Forge 1.20.1

When your hot bar is filled with other items and there is a custom item from his mod in your hot bar. Then when you pick up another item it replaces the custom moded item because in the code the item replaces "minecraft:air" so the game thinks it is a empty slot and replaces the custom moded item you worked hard to get.

Please perform isolation testing to ensure Simply Swords is the issue. I don't believe it is likely to be the cause