after a debug session with Juniper support, we found the problem: it's a bug related to the 15.1X49-D70.3 firmware version.
Basically the flow matched self-traffic-policy and SRX dropped packet because it not recognized the address as local.
Thanks to all,
Sebastian Wiesinger post was good but did not fully work for my set up. I needed to enable flexible-vlan-tagging as well as encapsulation flexible-ethernet-services on each interface in the mx104.
I also want to break down the actual configuration commands I ran to make the configuration more clear. Will also include connections to other switches that the ...