Yes, Whatsapp last seen can be wrong because the app may still be running in the background. This can result in your "Last Seen" is wrong, leading to an awkward situation.
So we highly recommend that you force close WhatsApp after exiting it to ensure that your "Last Seen" is always accurate.
If you want your "Last Seen" to be consistently accurate in WhatsApp, then you need to force close WhatsApp after exiting it if you wish your "Last Seen" to be always correct.