I was curious about that behavior so I did a couple packet captures on both our MX450 and MX250 Warm Space ( LAN captures )
MX450 MX 15.40 :
Uses the burned-in MAC for the VRRP MAC ( not following the standard in RFC 5798 )
MX250 MX 14.53 :
Uses a special MAC CC:03:D9 + End of the burned-in MAC of the device ( not following the standard in RFC5798 )
I'm a little bit surprised to see that both MX450 and MX250 are not following the RFC AND they are not even using the same mecanic for the VRRP MAC selection....