I made the mistake of filtering my Outline with the Only Selected filter once, and then that has been the default setting ever since. It’s annoying as I have to go in and disable it every time I load a map. I would be very grateful if someone knows how to fix this 
Hello there @Bobbo-King!
Since that setting is affecting all your maps, it means it got saved as the default value for your engine’s configuration.
To fully stop that, I would suggest fully resetting UE’s global settings. To do that, navigate to the engine’s AppData, usually at C:\Users[YourUsername]\AppData\Local\UnrealEngine, and from there, delete \Common\DerivedDataCache and \Saved\Config folders.
After that, re-open UE, and check the setting in a new map, the Only Selected filter should be disabled.