What you are describing in vague detail is not the graphics settings being altered, its the lack of VRam that Unreal is providing.
Currently, Unreal only will give the game 2GB of VRam to work with. When you fill up this buffer, textures will start to stream in and out in low detail settings, because there simply isn’t any place to store them. This effect gets worse when you throw Canvases into the mix, which you could possibly have 100s of different large images taking up VRam.
Unreal Engine 4.14 added functionality where the engine will use as much VRam as the graphics card installed has, so Tower Unite will benefit from this when we get around to upgrading to 4.14.