Wrong Apple ID associated with my last iPhone backup

I lost my iPhone 6 about a month ago and after giving up on finding it with Find my iPhone, I finally got a brand new one today. I am trying to set up my new iPhone with my most recent iCloud backup, but after signing in initially, when I choose my last backup a different Apple ID is associated with the backup - not mine. When I try choosing any other backup associated with my old iPhone 6 the same (different) Apple ID comes up. Could this mean that whoever has my old iPhone managed to bypass the find my iPhone lock and sign in with their Apple ID? Is there anyway to get my backup at all?

Thanks!