Like most other pump bottle adapters, there's a hole in the top of the adapter for air pressure equalization. The milk can leak from that hole if it's tipped too far.
Unfortunately, we don't know a way to prevent milk from leaking from the threads or air hole other than keeping it from tipping too far by not leaning too far forward. Try leaning back and see if it helps.
Reviewed by Diana West, IBCLC
June 10, 2021