Hello 
DiscoTOC style is connected to the .with-topic-progress class which is added to .topic-navigation if there are more than 1 post in the topic. So without reply the DiscoTOC style will broke.
I think TOC should also be active if there is no reply in a topic because it doesn’t related with the reply counts. On desktop it works fine topics with no reply too.
Here is a topic for testing: Customizing the topic list
I think that would be consider to change this behaviour.
It’s probably not a new thing but I didn’t notice it yet. Now we started it using more actively.
Thanks 
(iPhone/Safari) It’d be great if this could be fixed. Scrolling to the bottom of a long doc to get to the toc is a bit cumbersome.