I don't know how or why but this tends to occur when I manually disconnect from the game by pressing Tab and typing Disconnect.
The first Hero I created is an Apprentice and its always him who is put into my deck in the 1st slot. It ejects the hero I previously had in the 1st slot and moves it to an empty slot or off the deck. I cannot reproduce these results and this occurs rarely.
Just extra info that might prove valuable.
I noticed that this bug was much more common to me the moment I only had 2 Mystic in my Deck. They were in the 1st and 2nd slot of my Deck. I'm being very specific about Hero and Slot designation since it might assist with reproduction. I played about 6-8 hours back then when I was gearing my Mystics. I didn't bother reproduction with different characters or slot designations.
This reply is just to catch your attention to my other comment that I forgot to click Reply on.
This has been happening for quite some time, if we can narrow down a better reproduction case we can begin working on a fix for this.