This is the second time it happens.
The first time, one or two special events ago, the last card, legendary, disappeared at the end and I couldn’t loot it. At that time I thought it was me that I was distracted but today - HC Deep Special Event- I had the confirmation: it is a bug.
If you enter the following room by clicking the “free” button, without having looted the previous room loot, you automatically lose the second loot because it disappears! In this case it was the only reason why I run such kind of events: the purple chest.