I just played a battlegrounds game with the faceless manipulator anomaly where my minions on the board were receiving Rylak + Felemental tavern upgrades.
[Screenshot of final team](https://i.imgur.com/Y6hspMQ.png)
The above minions were never replaced with ones from the tavern. After each turn I would purchase a minion from the tavern, and each of them would receive ~100 stats from the previous turns Rylak/Fel combo.
I can’t remember for sure, but I think I used my first manipulator to clone a Felemental and complete a triple, then the next manipulator to clone the tripled Felemental.
Only realised halfway through the game that my mobs were getting stronger without me having to replace them with tavern copies.
EDIT: Forgot to mention was playing as Yogg