If they will not warranty, they will repair at reasonable cost. That's one of the best reasons for choosing Aerostich.
The seam tape is for comfort, not water sealing.
You can try ironing it down. You can try fabric cement.
You can help prevent seam tape from coming loose by following the washing/drying directions in the catalog and shipped with your suit.
One piece of seam tape on my Darien has come loose after only ten years and 100k miles.

It's the piece on the neck... I think lack of washing and body oil took it's toll. I bought a new Darien... may send the old in for refurbishing.