@KeithEB01 normally I would say welcome but it looks like you've been with us for a while, so instead congratulations on your first post, maybe... Anyway, just a couple of questions first.
When you say that it's "good for a while", how long is that while? A day, an hour, 3 -5 minutes, how long before it reverts to your screenshot?
Do you get notifications regularly? I mean do you ever feel like you get notifications late, or like you don't get them until you open the app?
Without more information it sounds like WhatsApp has been added to your sleeping apps list. This would cause the app to behave badly. You might only get notifications once an hour, or when you open the app, stuff like that. To check if the app is on the list go to Settings>Device Care>Battery>Background usage limits>Sleeping apps/Deep sleeping apps. Check both of these lists and remove WhatsApp if you find it listed here. If it is very important to you, you might consider adding it to the other list here, Never auto sleeping apps. As the name implies, these apps will never be put to sleep without your intervention.
Let us know what you find and if you found the app on one of these lists, and we can go from there.