To support the development of large worlds, all grid cells are initially unloaded. When the Level opens, the Editor will only load Actors that have their Grid Placement setting marked as Always Loaded, such as environment backdrops and managers. This supports the development of large worlds where it is impossible to load the entire map in the Editor.
In the World Partition window, you can manually select which grid cells to work in. Open the window by selecting Window > World Partition from the main menu.
In the window, click and drag grid cells to select them. Then, right-click the selection to open the context menu to load and unload the cells.
Here are what the Unreal official documentation says about the issue, why and how to fix.