On my Discourse installation, URLs to many topics are no longer automatically converted to links displayed as the topic title. I believe this behavior may have started when I had to make all categories private (see Staged users getting user_linked emails). However, all of my categories have the same permissions limited to the same group (trust level 1), so there is no security issue when displaying the topic titles.
The set of topics it still works for appears to be random; I haven’t been able to identify any pattern so far.
I’m at version 2.5.0.beta2 (eda8f319a2).
Possibly related: Private topic links are not displayed as topic titles