I experienced a weird character slot expansion bug as well. I already submitted a bug report in game but thought I should post it here as well.
I saved up enough gems to purchase a sixth character slot, went to the gem store and the upgrades tab was empty so I did a search for it. That brought it up with a few other items.
I purchased my slot, received the message about items coming in my mail, got no mail so logged out to check the actual character selection screen. I now appear to have 8 slots so it gave me a couple extra ones with my purchase it seems.