So this has now happened to me twice since it was nerfed.
Basically you cannot open the first 2 chest of the event. You can only open them after the 2nd event of the chain. Once you open them, the chest from the 2nd event of the chain disappear until you complete the 3rd chest in the event chain. After you clear the 3rd event the chest from the 2nd event reappear and can be opened. Once you open them the chests from the 3rd event disappear. After you clear the 4th event the chest from the 3rd event reappear and so on and so forth.
This repeats for the entire event. You are constantly 1 batch of chests behind which means you cannot open the final set of chests.
I was not at any sort of cap btw. I couldn't have been.