I think Safari v26 has actively broken multi-line text paste into text fields, in an EXTREMELY strange way.
If I copy an entire raw email message out of MailMate and paste it into a form on a web page, all linefeeds are removed and the first header line is case-squashed.
I can paste the same clipboard into a Terminal session or a BBEdit window and the text is unmolested. Pasting into hexdump shows a perfectly normal stream of all-ASCII characters with LFs