Lately since last patch SOMETIMES (NOT OFTEN, Maybe once every 10th game) I get up for .2 seconds a message that says "Blocked Input device" with a picture of a Mouse and Keyboard.
I only have Reasnow plugged into my PS4 so sometimes Fortnite finds out that I am using a Mouse&Keyboard, But only for a few miliseconds.
Might be worth looking into?