Its just started recently on my S21U-5G in whenever I press the recents button I get a message at the top of the screen saying "1 active in background" and when I click on the message a screen comes up with the title "Check background activity" and it shows "WhatsApp" with "stop" and "done" buttons.
WhatsApp is not open as an app when this is happening. If I click "stop" it goes away until next time I open WhatsApp.
If I stop WhatsApp in the background it still receives messages.
Has anyone any idea why this has just started happening as I have been using WhatsApp since I got the phone?