This didn’t used to happen, but now it does, even if you use the “reply as a linked topic” option.
Basically, if there’s any punctuation on the line with the link, including parentheses or a colon, even if there’s a space between it and the link, then the link preview will not be displayed. Just the link itself. This even happens if you are creating a “reply as a link to topic” and the punctuation is formatted by the discourse software itself. For example:
That worked fine. But now add a period at the end of the line