Hello,
Can you try try following command:
delete interface ge-0/0/10 unit 0 family ethernet-switching vlan members 51
Note:- Without removing above command, EX is able to assign vlan 51 to all incoming untagged frames but while sending out the frame belonging to vlan 51, it is still tagging it. Above command may enable the switch to send frames for vlan 51 untagged.
Regards,
Rushi