.mcpack, .mcaddon, .mcworld, etc. files will not load if the game is already open.

When opening a .mcaddon, .mcpack, or .mcworld file while the game is already open, nothing happens. The Minecraft window opens, but the addon, texture pack, or world is not imported into the game.

However, if I open those files (.mcaddon, .mcpack, .mcworld) while the game is closed, they are imported into the game normally, as they should be.