robots.txt: my new Sitemap line got glued onto the old one

Your robots.txt didn't end with a newline. Appending adds text straight after the last character, so the first new line joined the last existing one. Google now reads one broken address instead of two good ones, so neither of those sitemaps is discovered through robots.txt.

Split it back onto two lines and make sure the file ends with a newline.

Fixed with sed and all three Sitemap lines show separately now. How do I avoid it next time?

Before appending, check the last character and add a newline if it's missing:

tail -c1 robots.txt | od -c

If that doesn't show \n, run echo >> robots.txt first. Also check with curl after any edit, since that shows exactly what crawlers receive. Our audit tools include a robots.txt and sitemap checker.