If there are issues with traffic being lost, or packets that seem to disappear or never show up (or leave) an interface, try disabling Checksum Offloading as follows:
Navigate to System > Advanced on the Networking tab
Check Disable hardware checksum offload under the Network Interfaces header.
Then try to reproduce the problem. A reboot may be desired after applying this option, but it should not be necessary.
This has historically been an issue with Realtek NICs but has also been seen in some scenarios with Via Rhine NICs some specific Intel fxp chips, as well as virtualized/emulated NICs in some hypervisors, especially those that use VirtIO
The problem may also manifest as a PPPoE connection that will establish a login and bring up the interface, but will not pass traffic.