That's because the envelope timings are wrong. Specifically the Release Rate is way too fast compared to PCB/OST Recordings. I already made a bit of a
proof of concept by slightly decreasing the release rate which makes it slightly better albeit still little bit inaccurate. (And not only that but there are still other issues like the inaccurate FM emulation among others)
Edit: In case anybody's curious I only changed the following line in ymf271.c:
Code:
rate = get_keyscaled_rate(slot->relrate * 4, keycode, slot->keyscale);
I changed relrate * 4 to relrate * 2 but as I said, it's only a hackish workaround and not fully accurate