I have installed Raspbian Buster on serveral Pis (3B, 3B+ and 4B), the problem is everywhere the same:
If I config a WiFi connection, the Pi gets a DHCP adress from my router (AVM FritzBox), eg.: 192.168.178.35 (Router is 192.168.178.1)
If I connect network cable, the Pi gets a second IP adress but not in DHCP range, eg.: 169.254.56.161
Normaly I know, that when I plug the network cable in, the Pi gets a new DHCP adress from my DHCP server in my router and the DHCP adress from wifi and complete WiFi is disabled.
Now the Pi stay in Wifi and he gets a second nonDHCP adress for eth0.
Long time I didn´t see that, because I worked with WiFi and when I plugged cable in, everything worked but what I did´t see was the problem, that the Connection was also WiFi anf I thought, it was cable.
Many tests with new Images, different RasPis, changed cables, resettet router, and so on let me think, it could only be a problem of the actual buster Image (I´ve updated packages via sudo apt-get update && sudo apt-get upgrade -y).
Any ideas ??
Sorry for my english, ist not my mother language ….