For certain long messages in chat, the "see full text" link and/or scroll bar appears in normal chat transcripts, but not in a bookmarked conversation. For example:
Note that neither the "see full text" link nor the scrollbar for the second message is present in the bookmarked conversation, although for the first the link appears.
I'm not sure what the key difference between the two messages is.
Workaround is to click the permalink for the message missing the full text link to go to the transcript, where it can be viewed, assuming you already know that it's clipped: Since the "see full text" link and scrollbar are both missing from the second message, there isn't really any indication that there's more text there to begin with.
Incidentally, the presence of the scrollbar seems inconsistent, or at least is not predictable to me. I'm not sure why the first message has it but the second does not; the second has considerably more lines than the first.