Even when I explicitly quote a post the forum will eat it and I’ll have to edit the post to include the quote.
You can even see the forum post it originally with the quote then very quickly edit the post to remove the quote.
It’s a little more complicated than this, but essentially, if you use the small Reply button (from within a post) and then fully quote it, the system will automatically remove the quote due to redundancy.
Using the small Reply button puts an icon in the upper right of the post that can be expanded to see what was responded to, which is the same as quoting. Hence, why the system removes the quote.
If you don’t quote the entire post (break it up or just a portion of it), it should leave the quote intact.
Again, there’s more to it, but that’s the gist.