OK, this bug is positively ridiculous.
In Terminal.app, type this:
echo "zz\nz \nz"
...and hit return. This will output three lines of "z"s, with a non-visible space char at the end of the second line. It should look like this:
zz
z
z
Select all three lines (including the end-of-lines) and copy them. Now, open a new message composition window in Mail.app, and paste.
Notice anything? You only have two lines, and the second is z-space-space-z. The second newline has been converted into a space.
This failure works on just about any text IN TERMINAL that has lines ending in spaces, though there are some odd corner cases where it doesn't fail (like, the text produced by 'echo "z \nz"', if your Mail window is empty - though it will still fail if you have a line of text in there already).
Of course, this doesn't happen with other apps. In fact, if you take the text from Terminal, paste it into TextEdit, then REcopy it from TextEdit, when you paste it into Mail it comes in fine. So somehow this bug requires two Apple apps to be triggered.
I would be loving Ventura if it weren't plagued by so many stupid new bugs in features that worked fine before. :-(