I have an RPi 3B that has been doing a simple headless automation task for a few years. It has been connected via WiFi all this time.
Last week, it stopped responding and I realized it was not connected to the WiFi anymore. I then attached an ethernet cable to troubleshoot. The ethernet immediately got a DHCP issued address, but the WiFi also connected. I was not expecting the cable to have anything to do with the Wireless.
As long as the cable is plugged in (which gets its own unique IP) the WiFi works. I can SSH to the IP given to the WLAN interface until the ethernet cable is disabled or disconnected.
Not sure where to start on this, but guessing this may be a question for someone far more pro in Linux than me. Personally, I have never seen this behavior before.