The your topic is similar to… banner:
Has class="composer-popup hidden similar-topics ember-view"
This kind of clashes with an assumption that a dev on our side made, namely that everything with .hidden should be … hidden 
So he put a
.hidden {
display: none !important;
}
in the CSS somewhere.
Could be nice not to put class=hidden on things that should not be hidden
Hmm not sure, perhaps @Johani could have a look… low priority.
Indeed, didn’t expect this to be a Prio 1 
Is this addressed in a bug somewhere?
I am running a new Discourse 2.6.0.beta2 at the moment, and the “similar topics” view never shows up.
Checking with dev tools, the request to load similar topics is made, and the similar-topics div is populated with content. It has the class hidden however, leading to display:none !important; through helpers.scss.
So my users won’t ever see similar topics, resulting in duplicated topics here and there.
Just found this commit, that apparently broke this: https://github.com/discourse/discourse/commit/5a34c7cc9e50df8965406e0c3112793313461de3
The change is under discussion here: Guest Gate (Sign Up Popup Plugin) - #55 by michaeld
I can repro this issue here on meta as well, seems like it is indeed the same.
I think that should fix it:
https://github.com/discourse/discourse/pull/10701
Awaiting review on our side.