1. Had a TPE doll lying on its back in its box and exposed to house temperatures etc. for about 5 months. No damage and the butt is still very smackable.
2. She might get a flat butt if you don’t pad it with memory foam and stuff.
3. Please take her out and check her over before putting her in storage–otherwise it might be more difficult to handle damage/configuration issues via the vendor/seller/manufacturer if several months have passed.
4. If your doll has the standing feet option, you can consider storing her vertically to take weight off of her back and hind end. It might help avoid any flattening of the TPE.