When you want to completely remove all traces of your previous WhatsApp account from a phone, it's not a matter of doing so within the WhatsApp app itself. Go into your Settings >> Apps menu, find and open the WhatsApp app entry, tap on the Force stop button, then find and tap on Clear data, and then tap on Uninstall.
Using Clear data wipes that app's settings and configuration data, along with its cache. With some apps, just doing Uninstall does not necessarily mean everything relative to that app gets wiped. It will remove the executable parts but the config data will be left as is -- i.e. if you reinstall the app later your account info is still retained.
It sounds like you just used Uninstall and then when setting up your new number this created a mix of your two different accounts. Keep in mind your with a lot of online services, like WhatsApp, your account info is primarily stored online (server) and your phone is just fed data to be shown to you (client). Your WhatsApp app still works with whatever data is retained on your phone but it still relies primarily on the constant exchange of data with your online account.