I agree with Stefan. Unfortunately most of Juniper's MC-LAG documentation is either unclear or missing this important piece of information for that for all L3 IRB's that span the 2 x MC-LAG Core Nodes, a static ARP associatation must be configured. Stefan has provided the CLI commands which must be set of each Core Node, with each side pointing to the associated MAC of the other side, with matching IP's (like 2 on one side, 3 on other, and 1 being VRRP).
Work is being done to improve the documentation in this area. BTW, if you use [later/latest] Network Director to configure MC-LAG and VRRP, ND will automatically create this association in the config. I think with ND 2.5 and beyond, and proper product SW. This may only apply to EX9200 at this time as well.