Ok i figured it out @Tree
Seems when you have multiple secondary tags, like your post which has two - System and Worldbuilding it doesn't display in all discussion
This might be a bug, as i cannot configure this. I can however configure a limit of how many secondary tags can be put on a post (including requiring at least one if I want)
I can submit this as a bug report, but being free software there's no real promise it will get fixed in a timely manner. What we can do, is maybe restrict posts to only have a single secondary tag, and maybe do a pass on what the secondary tag options are such that they make sense to not be needing to use multiple. Like - are the ones we have broad enough?
What do you think?