Two different thing to notice :

- WhatsApp relies on a background daemon "com.whatsapp" running to handle the messages. You are technically online as long as this daemon is running, and there's no official way to kill it from the WhatsApp interface.
(According to the rationale from some WhatsApp FAQ is that they want to mimmick the "always on" feature of SMS. You can't switch SMS online, therefor you shouldn't be able to shut down WhatsApp either).

If you want WhatsApp to stop processing messages, you need to kill the background daemon.

If you only close the app (kill the card, press "back", etc.) you're merely closing the interface, not shutting down WhatsApp.

The fact that it suddenly doesn't show online anymore on Android is probably some side effect of how that peculiar version of Android (or some addittional tools) handle rogue processes / ressource conservation. But that's not the normal expected WhatsApp behaviour according to their docs.


- Sailfish OS is a true multi tasking OS. All the open cards are always running (unless grayed out). But most 4.4 Kitkat era applications (=the API level exposed by aliendalvik) don't have a concept of application "still running but not in focus".
Thus nearly no android app knows that you swiped to switch to another app, and WhatsApp is among them.
Because it's interface is still working, it's still believing to be infocus.

The only useful tip that I can give is to get out of any message screen back to the main contacts/list-of-chats screen, before switching to another card.
At least WhatsApp won't be thinking that you read a message while the chat is open and won't mark as double-blue tacks (read) in the senders' app, but only as double-gray tacks (delivered).