The additional loot on explorers is indeed the announced secret replacement, as can be read in this post
Name: Explorer Week
Runtime: 2 weeks (Starts after the deactivation of the Mini Adventure Mystery Box)
Description: During this time all explorers have the chance to bring additional loot. Isn't it a great opportunity to have a look at the fast Lucky Explorer? He is now available in the shop.
If you feel that you didn't get the star coins you were supposed to get, please contact Support via the contact form on this page