@madbrain I rebooted one more time, and the problem went away - Verizon gateway no longer showed. Very strange. It's an intermittent problem.
I decided to remove the USB Realtek 8156B NIC, and replace it with a PCI-E Intel I-225V (B3) that I bought at Central Computers earlier today. This necessitated shutting down the machine to insert the PCI-E card.
During the next boot, this is what the interface assignment screen looked like (MAC adresses omitted) :
58150e9e-53b2-46dd-81e9-09f2c62e0d3d-image.png
As you can see, both the Comcast and Verizon interfaces are assigned to the same ix0 network port. Previously, Verizon was assigned to the ue0 network port for the RTL8156B. That port no longer exists since the USB NIC was unplugged. It is a bit disconcerting to see two interfaces on the same NIC, to say the list. Not sure what the right behavior should be, though. Maybe have the interface be unbound (no network port) ?
So far, I am not seeing the same issue with all 4 Ethernet ports using Intel NICs (motherboard 1 Gbe, X550-T2 2 x 10 GBe, I225-V (B3) 2.5 Gbe), but it's only been a few minutes.