Managed to suss this, so for anyone else having issues you need to strip any newlines or tabs from the output generated from ckeditor.
This worked for me:
$string = str_replace(array("\n", "\t", "\r"), '', $string);
🏷️ Limited Offer: 25% OFF 🏷️
First 100 purchases only — Special discount!
Use this coupon on checkout: PHPDXPJ_D1SC1N5T
Get it now