Sometimes I have really short first posts like only two words. This ends up harming the meta description. It would be best if this could be detected and if there is not enough content in the first post, some content from the second post is added to the description
I just noticed this also is very problematic when the first post s just an image or media video
What if the second post is a short response? Would you expect the 3rd post meta description to then be used? The 4th also if that one is too short?
Personally I think the short posts shouldn't be worried about. Better to focus on ensuring there are more quality initial posts on your forum and encouraging members to make their first posts like this. The best way to achieve this is to model it plenty of times with your own threads.
You can't control members to that level. Especially hard with big boards.
A better option might be for XF to detect a short post like when it's just an image or media and append the thread title and forum into the description to give some content. Having an image or media tag be the description looks terrible in google.