mail.app – Applescript’s “Set first paragraph to” using Mail no longer works with 13.1 (Ventura)

I have an Automator/Applescript task working since months that takes a selected e-mail, replies to it and adds a standard greeting on top of the reply.
Since updating to MacOS Ventura (13.1), the script still works, but now the added standard greeting clears the rest of the mail before being added (instead of adding the text on top of the replied content).
This is the relevant code:

    tell application "Mail"
        delay 1
        activate
        set theMessages to the selected messages of the front message viewer
        set theMessage to first item of theMessages
        set theOutgoingMessage to reply theMessage with opening window and reply to all
        tell theOutgoingMessage
            set first paragraph to "Hi, "
        end tell
    end tell

So the result now is that the message only consists of “Hi, ” instead of “Hi, ” followed by the rest of the original e-mail.
When going through the script step-by-step, I can see that the original replied to mail content is there, but it get’s cleared as soon as the “Hi, ” is being added.

Thank you for any assistance.