That’s probably because your graphics card’s memory is getting overwhelmed, so Unreal lowers the texture resolution to prevent this from happening. Some people have fixed this by doing the following:
- Locate the game’s .exe file
- Right-click it and select Create shortcut
- Right-click the shortcut → Properties
- In the Target field, add a space at the end and then: -dx11
Apply and run the game from the shortcut