Surface Book 2 user here. It seems it isn't just a Surface Dock issue. I have had 2 other dongles purchased off Amazon, USB-C and USB-A to ethernet (with additional other types of ports and etc though) and they both disconnect and reconnect continuously, anywhere between 15 seconds to a minute each time, sometimes even after 5 seconds of reconnecting. I thought my USB-C dongle had gone bad so I tossed it in the garbage and purchased a USB-A one. Turns out it wasn't my dongle and I threw away a $50 dongle for no reason. Different ports, different dongles, different networks...it just keeps disconnecting.
I have just purchased a singular USB-A to ethernet (with nothing else on it) to see if the problem persists but haven't gotten to test it yet. I have a regular 4-port USB hub currently connected and it doesn't disconnect. I use another dongle in my office with dual USB, dual 4k HDMI, thunderbolt, and USB-C PD and that doesn't disconnect either. It seems to me the ethernet portion of these hubs/dongles/docks are causing these annoying, constant disconnects.