In reviewing my prior comments, I saw that a quote I had included in one of my comments looked odd, and realized that I had forgotten to put a blank line between the quote and my reply to it, as such:
Assuming the intended behavior is for the quote to continue until there's a blank line to end it (I'd think a line break character ending it would make more sense, myself, even though I think a comment looks better with a space between quoted text and non-quoted text, but I assume there's a predefined means by which Lemmy clients are meant to display them), the preview formatting should highlight the full text that ends up being formatted as a quote.
Testing the same behavior with the heading formatting results in the intended behavior, with the heading formatting cutting off with a line break, though I'm not sure if headings are meant to function in the same way or not.