I saw something similar at a customers Shopware 5 store.
After purging the account the bot immidiately created a new account.
Also subscriped to the newsletter in high frequency.

The used mail account was an account which doesn't accept mail e.g. example.com so the mail server always send an error report to the customer who was restricted using his mail system effectively.
I added honey pots to both registration and newsletter subscription to stop these attacks.