If you close the app in recent apps and open again it reloads your contacts. The problem is that MIUI only lets WhatsApp to have access to contacts when in use, so the app staying in background looses permission and doesn't reload your contacts when you awake it. It needs to have permanent access and not only when at use.
Enviado do meu Redmi K20 Pro através do Tapatalk
Edit: it's commendable the way Xiaomi wants to change the way apps have permissions to your device, but it needs to be well done, ie, users should have all the options they want, in the end it is a consumer choice. WhatsApp it's the only one suffering from this (from my experience so far with the rom) it needs permanent access to contacts and not only when it is used. I don't think Facebook will change WhatsApp because of MIUI behavior, so it's on Xiaomi side...