Hello @gkaufmann,
As @tialaramex said, seems you have another cron job out there renewing your cert every day. As you installed certbot using the debian package it is worth to check this post I wrote a few minutes ago because maybe you have an active systemd timer that is trying to renew your cert… but well, it should not renew it if it is not due to renewal yet…
Cheers,
sahsanu