Here's an example post:
https://puddle.town/@null/113004193514274927
The actual link at the end of the post works fine, but the preview text is German (I think) and if you hover over it, the URL is https://www.youtube.com/undefined.
It appears this way for me from two different networks, and doesn't seem to matter if I'm signed into Mastodon or not (i.e., I can reproduce it in an incognito/private window).