This is a knwn behaviour of JDHCPD: https://kb.juniper.net/InfoCenter/index?page=content&id=KB28646
Looks like you're trying to change IP before lease expires and arp will be permanent until then.
I remember seing a place where you can disable this behaviour of JDHCPD but I cannot find the reference/documentation. Maybe someone else in here can confirm og deny this functionality being possible.