Hello,
I checked this:
First check on d5f6262c4f which is about 2 days earlier then your Discourse version.
Here is the listed Welcome topic.
Now I unlist it.
No found, so this is works as expected on this version. Now I update to the latest version Discourse which is now the b75dc04a25.
And it still no found so I cannot repro this on the very latest Discourse. I think you have to try to update to the latest version. 
Just to be sure I list it again
and unlist…
