I have noticed an issue (or inconsistency?) with luck and I think I know why.

For this example I will be using the boss Swamp Hag from the campaign Swamp Croc Feud in the map McCroc's Still Farm.

I am using the items: Black Hearts of Love along with Greedy Gazer which add up to a total of 100% Luck.

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When killing the Swamp Hag you mostly get items that re-roll, but on occasion you will get Trash rarity items that don't reroll into a higher tier (Common or above)

And I think I have a reason as to why:

Swamp Hag's loot table does not contain any Common weapons, so the game thinks there is nothing better to reroll into; I have never dropped an Uncommon weapon while using 100% reroll which makes me assume there is not common weapon to reroll into uncommon in the loot table.

Just wanted this to be confirmed as a bug, that hopefully gets fixed, or working as intended as you currently, from what I can see, cannot reroll an item if there is no direct higher rarity.

Thanks