Event and Mailbox

Over the year I have a bunch of item I was saving in my mailbox, Like old war front box etc. Main reason being I don't want to use My banking space to store them. Today (11-05-19) I ran in to something, might be because of the event and log issue.

I was picking the reward in the mail box to go do IU, when I click the button to get my item, it happen that at the same time I have received the Items for today event. The mailbox message window closed and the mailbox reset with the new item I just received, but instead of having the IU item in my inventory, the 1st 5 item in my mail box where taken out and put in my inventory instead.

I don't think I could replicate this but i'm more concern if it had been timed items with expiration time.