BUG 1
From what I've seen, the intent of the buff is to offer a 2nd event loot slot provided the explorer doesn't have an equal or higher amount of native loot slots.
So I checked a couple of returns, and a number of explorers only returned with 1 eventloot slot
Both loots are from Lovely explorers doing a Short treasure search
(Correct) Expected:
https://prnt.sc/p4M9mxJcmoDH
(Incorrect) same explorer type
https://prnt.sc/Qs7cvqEByIQp
Both explorers have the following buffs active:
https://prnt.sc/gSiZIt6swk1Y
+
https://prnt.sc/ATFcbErZV3U7
I checked a complete return batch and there were multiple instances where only 1 event-loot-slot was returned instead of the minimum of 2.
BUG 2
Both explorers in the screenshot are Lovely explorers, which has a native skill to always return the highest amount possible of any loot.
This skill (and skills like it) don't seem to apply on the bonus loot slot. See the first image of a Lovely explorer.