Symptom: Xbox One controller connects initially to the wireless adapter, but after turning off or disconnecting they will not reconnect. Requires a reboot to allow controller(s) to reconnect.
1. Close Steam until the controller is connected, then restart Steam (if desired.) This was my and the majority of the people with the issue’s solution – See Gabriel Barsali’s post on the 4th page of Xbox One Wireless controller keeps disconnecting.
2. If you have the newer (Xbox One S and later with Bluetooth) controllers, try removing the Bluetooth Xbox Controller adapter from the Bluetooth & other devices window in Settings/Devices. If you disable Bluetooth (or remove the Bluetooth dongle) before removing the Bluetooth Xbox Controller Adapter it will appear “grayed out” in the Bluetooth & other devices window; you can’t delete it without Bluetooth on. (Of course.) This was not my issue, but is a potential solution listed by others.
3. Some feel uninstalling GeForce Experience is required. I have GeForce Experience running and this did not cause a conflict for me. It was definitely a conflict with Steam and the new Steam capability to manage non-Steam controllers.