What exactly does this new line do?
I am asking cause I got players who say that Raidables have less loot since the latest update.

it prevents players from adding or removing items from boxes and TC. this has always been enabled but was only recently optional.

Would any of the new options been able to change something about the loot that was set before anyway?
I'm not sure if my players are seeing this right.

items can move into lockers, and that amount will not be shown in the UI unless lockers is added to Additional Containers To Include As Boxes. adding lockers to this list will require they be looted before the raid can end. you can disable Move Items Into Lockers to prevent both.

items stack in the TC. so if 12 items spawn and 6 of them stack then it would count as 6 items because it is quite literally 6 items then. you can disable Move Resources Into Tool Cupboard, but you cannot prevent stacking in the TC

only the server console message should show the amount from the Amount Of Items To Spawn option. the UI will always differ based on how items are distributed into boxes, TC and other containers using your config and profile settings.