UDP routing issue to PPPoE client

[marcoarrigo]

I'm experiencing an issue in the UDP routing that is making me crazy ....
On the same physical interface ( a 10GB/s port), splitted in different vlan I have 3 interfaces.

vlan 100 on which a PPPoE server works
vlan 177 where a SIP and DNS server are connected to
vlan 275 as wan interface

as you can see in the following picture the tcp packets send by a PPPoE client on the vlan 100 are correctly routed to the vlan 177 and the server's answer correctly "come back" to the vlan 100.
What I can't understand is why sometimes (not all the times) the udp packets are sent by a PPPoE client on the vlan 100, correctly routed on the vlan 177 while the answer is routed by the PfSense on the vlan 275 and not on the PPPoE interface.

Have you any idea?