Hey @Tinytot87
I'm sorry that you are still experiencing this issue with Assassin's Creed: Valhalla.
Could you please try this procedure:
1. Press Windows key and i key together to open Settings.
2. In the Settings window, click System.
3. Then click Display tab from the left panel.
4. Then choose Advanced graphics settings.
5. In the pop-up window, uncheck the option Enable fullscreen optimizations.
Please let us know if, after trying the procedure mentioned above, the issue persists.