I'd argue this is a feature and not a bug. When copy-pasting text from PDFs, I'd love to not have to deal with unicode and ligatures. There's another comment upthread here where someone's complaining about having to deal with unicode.
If Preview can do this automatically, please don't change that feature.
I think the GP commentor meant that the ligatures are converted lossfully into an arbitrary substituant character (e.g. fl -> l), rather than that they’re taken apart losslessly.
If Preview can do this automatically, please don't change that feature.