How to prevent switching app or locking screen from terminating current WhatsApp working task?

The problem is like this: 1. When I open Whatsapp 2. Select an attachment in a conversation 3. Forward this attachment using "Share" button sharing to Google Drive app 4. Select the destination 5. Waiting to finish. If I have initiated the forwarding action with a lot of files, I need to wait until the whole process is completed. Or if the screen go is locked automatically or switching to other app, any action leaving the current working screen will trigger the Whatsapp back to step 3. Is it unavoidable in iPhone? Is it Whatsapp development problem or iPhone built-in limitation?