It’s in the og tags of the website in the header, e.g.
A well specced social media platform (Discord, Facebook, Discourse etc.) crawls the page of the link and enriches its local presentation.
That crawling code is the responsibility of the hosting platform (ie Wechat).
In this case Discourse is only serving the page when it is requested.
In this case there is nothing left for Discourse to do - it doesn’t have control on how WeChat presents things locally - how could it?!?
There is also a possibility there is an access issue between WeChat and your server. You could also explore that but you will never know the exact route it’s trying to take because you don’t have those priviliges.