I’ve run into that issue with the bugged boss and minibosses. Often fast traveling to the start or dying and starting over will solve it. The main culprit seems to be Tyrone. If I damage him too much the first time he drops down, he stays at zero health and won’t die unless I die or fast travel to the start to reset Joey’s arena.
Also I have noticed that lower Mayhem levels seem to drop dedicated loot more often than the max level. I had this issue pre Mayhem 2.0 and it seems to still exist to varying degrees. Almost everything I spend hours farming on the old M4 levels, would drop on the first or second run if I dropped to M2.
I haven’t done as extensive testing on Mayhem 2.0, because the power difference is so vast, but I imagine that a lower Mayhem level would still yield equal or higher drop rates.