Do you mean that when you click on the show, the tag you're looking for doesn't show up underneath Genres: in the Tags: section or in the entire tags list for that show? Do you also click on the link next to it where it says (Vote or add tags) because some tags that are added later or haven't been voted on enough, are hidden and will only show up if you click on that link.
I've checked all from the first page on this link on two different browsers, following the sismance tag: https://mydramalist.com/search?adv=titles&ty=68&th=366&st=3&so=top and all of them have the tag there, but not all of them have it shown underneath the Genres: line. You have to click on (Vote or add tags) to see the tag.
Examples of the ones that have the sismance tag hidden form the main page:
Go Back Couple - https://prnt.sc/tc4sro
I Hear Your Voice - https://prnt.sc/tc4t12
Goblin - https://prnt.sc/tc4tnf
The King's Avatar - https://prnt.sc/tc4u1r
cat chan:Yeah, that is what i am telling in goblin there is no sismance tag
I'm sorry, but now I'm not understanding you. ^^;;
The tag is there, just not listed on the main page. That's no bug. There are too many tags and they can't all show up underneath, 'cause some shows can have more than 30 tags, so the first ten show up on the main page, and the rest are hidden. It will look messy if ALL the tags are shown on the main page. Just click on the (Vote or add tags line) [like here - https://prnt.sc/tcapq3] and the rest of the tags will show up, like the sismance tag that you're looking for.