Can you double check that you have no routines that affect the state of the Wi-Fi? Also, ensure that Intelligent Wi-Fi is either off, or you have your frequent Wi-Fi networks on the list.
Regarding the battery drain; check the battery stats in Device Health and check what apps are responsible for utilising resources during foreground and background, Sleep/Deep Sleep apps accordingly to their usage; if its an app that you don't rely on, deep sleep it.