Simple bug, as stated, the game has started exclusively opening in windowed mode despite me never having played with anything except fullscreen since 2016 (never windowed, never borderless).
This started happening over the past week or so from my recollection, and it is 100% consistent. I have had it happen before on occasion, but now it’s 100% consistent over multiple days, which is new to me.
Overwatch is the only game this happens in. I have no interfering programs or overlays that could cause it.