Welcome to the Let's Encrypt Community, Youssef 
Looking at the certificate history for rocketmeup.com, we can see what's happening here.
I'm assuming this is referring to this certificate expiring 2021-05-31 that covers:
X509v3 Subject Alternative Name:
DNS:rocketmeup.com
DNS:www.rocketmeup.com
I believe this is referring to this certificate expiring 2021-08-02 that covers:
X509v3 Subject Alternative Name:
DNS:rocketmeup.com
As you can see, the newer certificate does not cover www.rocketmeup.com and thus is not considered a renewal of the older certificate. This is why you received an expiration email for the older certificate.
You can find a detailed explanation here: