Receiver Mail not sent when sender mail is sent

[ja-sk]

I have a form that sends emails to predefined recipients based on a form field {recipient}. This functionality works perfectly as long as I don't enable the sender confirmation email (by leaving the "Send confirmation to sender" checkbox unchecked in the form settings).

However, when I enable the sender confirmation email, the confirmation email to the sender is sent successfully, but the email to the predefined recipient is not sent. In the backend "Mail" tab, I can see the intended email to the recipient, but there’s an issue with the subject line. The subject, which I’ve configured as "Mail to {recipient}" for internal tracking purposes, does not have the {recipient} variable replaced as it does when I do not send confirmation mails. This makes me suspect that after sending the confirmation email to the sender, the form’s configuration somehow gets disrupted or lost. Anyone else with this problem? How can I solve this?

I am sending via authenticated SMTP on PHP 8.2 and TYPO3 12.4, powermail 12.4.4