I’ve sealed mylar with a vacuum sealer, but it’s not easy and requires putting the mylar bag inside a sealer bag then sealing the whole works and then removing the mylar....a lot of trouble. Mylar works best (as far as I know) with o2 absorbers. Seal right and left with a small opening left over, then drop in absorber and seal middle...with hand sealer iron.
What I was trying to figure out if it was due to silver.
Lots of the mylar has a dusting of silver to a full coat on it.
Silver is an excellent anti-microbial. If that is the reason for it working so good, then maybe you could just get silver doped PET instead.