The sort by replies / last post : Clicking on text is always DESC. thread / author is always ASC !!!
Clicking the round arrow icon works fine though ...
This is unchanged from 1.8. The only difference is there is an arrow instead of [asc] and [desc] text.
jsoltesz.com - MyBB lead designer and team member since 2006
Then this is an issue to be reported for 1.8 as well ...
I’m not sure it’s a bug.

When you go to sort by thread title or thread starter, it makes sense to sort alphabetically ascending.

When you go to sort by replies or last post, you probably want to see the top ones first.

It is also interesting that in the 13 years since MyBB 1.0 was released apparently no one has commented on this until now.
jsoltesz.com - MyBB lead designer and team member since 2006
We should consider and focus on the highlighted matter.
Its irrelevant to the fact that it has not been observed for 13 years neither it proves that it is correct as it is just because of no one noticed it.

What is the logic behind carrying over initial state in URL, making the text clickable and return the same sorted list on clicking the text?
You click on the text to sort by the selected item, e.g. thread subject, thread starter, replies, last post.

You click on the arrow to change the sort direction.
jsoltesz.com - MyBB lead designer and team member since 2006