You will need to code the client to restore all sprites upon ALT TAB back in. As once you ALT TAB out from client, Windows 10 stores all the sprites in black and white so you wont waste resources, and then upon ALT TAB back in, the sprites are not restore or reloaded.
Understand how the sprites are being loaded up upon loading, and try code a refresher to reload sprites upon client is ALT TAB back in.