Did you use the little plastic O ring? I replace that every time I swap out my cylinders. You can get like 6 of them for $1 at the welding supply shop.
One thing is 100% sure - there was no leak - I submerged the bottle in my brute to see if the plastic O-ring was finally airtight.
I'm a little surprised myself - it must be a bad filling.
Thanks god for my 5lbs emergency bottle