Sending multiple emails, but only receive 1 single SNS delivery notification.

I send emails using PHPMailer, vie a foreach loop, using SES SMTP credentials. Batches of about 20-30 every few days. This has been running for a number of years on php5.6 Recently, I upgraded to php7.4.

Since then, the same php script run, with no errors and all emails are delivered. However, I've noticed that I now only receive a SNS delivery receipt email for the first email sent in the loop, and SES dashboard only shows 1 email as being sent.

I'm not quite sure what would cause this behaviour and weather I should be worried or not.

Any input appreciated, thank you.