I sent in a bug report for this, but I wanted to post it here, as well, to see if other players can reproduce (I only had one opportunity, since I have to go to work).
Bug: If you use a Total Biscuit of Everlasting Will while you have a Total Biscuit of Rejuvenation in your inventory, the Reju Biscuit will transform into an Everlasting Biscuit.
As far as I know, this is only possible with Kleptomaniac (only way to get Reju Biscuits, now). I was experimenting with how many runes Ez can abuse with his Q (a lot), and came across the bug. I had received a Reju Biscuit in item slot 3 and an Everlasting Biscuit in item slot 6 (I'm not sure if the Biscuit was received through Kleptomaniac or Biscuit Delivery, though). I decided to use a bunch of the items to clear my inventory to get more stuff, and used the Everlasting Biscuit in slot 6 first. I know I used that, and not the Reju because I was checking the mana to see the 40 increase in the cap. After consuming, I found that slot 3 was now empty, and slot 6 now had another Biscuit labeled as an Everlasting. Upon consuming this new Biscuit, my mana cap increased once again, so it was definitely an Everlasting Biscuit.
Another bug related to this is that no items earned through Kleptomaniac are shown in the Scoreboard during the Replay, so I couldn't go back and try to record it that way.
Another possibility - because I wasn't paying attention to the game time - is that I just happened to get a new Everlasting Biscuit from the Biscuit Delivery Rune right after eating the first, but the Reju Biscuit still disappeared.
If anyone else can test this and reproduce it with video to verify it, that would be helpful. I'll be attempting to do so this evening, after work.