When you play an Aviana, Flobbidinous will reduce to a cost of 1. However if you play another minion he will copy that minion and go back to a cost of 4. With Aviana being a constant Aura effect she should always grant him the ability to cost 1 after transformations.
This bug started two months ago. I already posted it here: Floop interaction with Aviana https://i.imgur.com/lLlurPu.jpg
No fix for the bug yet.