WhatsApp “Unofficial App” Error on Oppo A15

I use an Oppo A15 and have been unable to use WhatsApp normally because I constantly receive an “unofficial app” error whenever I try to log in. This problem has been ongoing since around October 2025. Each time I log in, WhatsApp shows that I am using an unofficial version, even though I am not. To solve this, I have repeatedly uninstalled WhatsApp and reinstalled it both from the Google Play Store and from the official WhatsApp website, but the same error continues. I also reached out to support and was only advised to download the official app, which I have already done several times without any change in the situation. I request that this issue be properly investigated and fixed so that I can log in and use WhatsApp on my Oppo A15 without receiving the “unofficial app” warning, and so that my number can be used normally again.