On my Mac, I have the network service order set in this order of priority, Ethernet first, then Wireless. I work with both connected at the same time.
Yet, I noticed that even when I'm on ethernet, when I turn off my wireless service, I lose network connectivity for a few seconds.
Shouldn't the Mac automatically use Ethernet as the main network connection, even if I was already on Wireless, then plugged in the Ethernet cable?
BTW, I support a workplace full of Mac laptops and have seen this on those machines too so it's not just my computer.