Hmm, is your drageable element a child element? If yes then instead of half of the page width, try half of the parent element’s width.
I experienced the same thing for my app (which is designed for desktop use first) and this fix works for me but it does have some quirks depending on the parent-child layout.
It’s not a bug persay as the offset is very consistent so it’s about figuring out what works for your situation.