Hello,
This is not supported. There are workarounds:
1/ use VRRP on this unit, no need for VRRP backup. Then this unit will accept 2 dst MACs: one from phys.interface, and VRRP VMAC.
2/ use stub bridge-domain with IRB instead of L3 subinterface. Changing MAC address on IRB unit is supported.
HTH
Thx
Alex