If you reply is the next post, it doesn’t show. it is assumed to be a reply to the previous one
If there is a post between the post you reply to, and yours, then it shows the little reply indicator.
Discourse has always been this way
Similar to how it removes it if you quote the entire post.
If you quote the entire thing, it’s redundant. Just being a reply, with no qualification, implies you are implying to the entire thing.
It’s their way to remove unnecessary bits in the interface that either cause more things to render, or unnecessary clutter