The rare fruit whose main research interest is an enzyme, not a polyphenol.
Pineapple is one of the few common fruits whose core research interest isn't a polyphenol or vitamin, but an enzyme — bromelain, found throughout the fruit but concentrated most heavily in the stem.
Bromelain is a protein-digesting enzyme that's been studied for anti-inflammatory effects, particularly around reducing swelling and bruising after minor surgery or injury, and for easing symptoms of sinusitis in some clinical trials. Most of this research uses concentrated bromelain supplements rather than the amount found in a normal serving of fresh pineapple — a familiar pattern by now across several foods on this list.
Bromelain is also why eating a lot of fresh pineapple can leave your mouth feeling slightly raw — the same protein-breaking action that makes bromelain interesting for inflammation also affects the proteins in your own mouth tissue. This effect disappears once pineapple is cooked or canned, since heat deactivates the enzyme.
Fresh is necessary if bromelain activity specifically is what you're after — canned and cooked pineapple are still a reasonable source of vitamin C and manganese, just without the active enzyme.
Concentrated bromelain supplements are sometimes used post-surgery under medical guidance — that's a different question from fresh pineapple as a food, and worth discussing with a doctor rather than self-directing around a procedure.