Many times I have clicked the “quote whole post” icon when writing a reply. When I click the red button to save and post, the quoted text disappears. Happens often so I have to go back and edit immediately to re-add the quote again.
If your guote is immediately after the post being quoted, then, I believe, the forum SW concludes it’s unnecessary. I don’t think this happens with partial quotes as the partial quote might have some specific meaning.