I think it might have started some weeks ago: gamepad would work fine either via bluetooth or wired connection. Now it works only via bluetooth. To be specific: it connects and works via bluetooth while also charging, but if bluetooth is turned off then the gamepad charges but is otherwise invisible to the system-- it only appears in Steam settings or System Settings menu of KDE Neon when connected wirelessly, even if charging by wire at the same time.
At this point I am wondering if the pad working via USB was a confused, false memory of mine. It definitely worked both ways before when I used Mint a few months back, though I had to jump through a few hoops to connect it via BT--Mint's GUI for bluetooth didn't work properly and bluetooth pairing was to be done through terminal commands. After installing KDE Neon I was overjoyed to find out that its GUI worked as it should... but now I wonder if I failed to notice non-working wired connection from the start. Is it a known issue, perhaps, with PS4 controllers or other devices?