Maybe you've already found your solution. Reading Spendlove's suggestion I wanted to add: peel the backing away while it is still hot from the iron. Hope you let us know what works.
Let us know what you do to remedy this wrinkling. We are all interested. Thanks.
If you iron it, it will soften the adhesive and then you should be able to peel the backing away. Iron it again with some kitchen paper between it and the iron and most of the adhesive will be removed.
Why not try that on one corner?
Could you ADD a new back?
I was thinking to rip it apart and sew a new back but still needed someone to give some thoughts on it. I can buy a backing all in one piece when the time comes to fix it. I will try both methods. It's a rabbit on quilted material and too cute to just discard. After I finish I give to someone who really needs one. Thank you.