It seems like there is a bit of a bug in the chest mission completion coding.
When all of the chest slots are full and you attempt to put a new reward into a mission with a follower you have to choose which full slot to replace. You get something like this.
Which game mode was this, or which specific battle?
Been trying to replicate this on my own account and after I have spent the food to finish the Minion mission to open the chest I am replacing, I am only getting the pop up to open the Chest.
It’s not going back to this screen like you have experienced so far
Game will inquire on which Minion mission to buyout with food.
Choose any chest to buyout.
Game will refresh the buyout screen and ask for 4 additional food to buyout (as pictured above).
Additional observation:
It’s a timing glitch. When at the 4 food buyout screen, you can exit the buyout screen and return to the chest selection screen. Then, you can go back to the chest that experienced this glitch and it will now show as “Unlocked” and can be opened freely to make room for the newly dropped chest.