The pitch of OPL2 instruments in S3M files sounds noticeably off when compared to OpenMPT, Schism Tracker, and Scream Tracker 3.21 in DOSBox. This affects instruments regardless of whether they use pitch bends/portamento effects. It is most noticeable with "distorted" instruments like electric guitars.

Here is an example recording.

The first part is the correct sound as heard in trackers that support OPL2 instruments (OpenMPT was used in this case). The second part is the same S3M played with AdPlug. I have tested both the Winamp plugin and the AdPlay DOS frontend, and the issue is present in both of them.

The issue persists even when AdPlug is used to log the S3M to an RDOS RAW; the resulting RAW will also end up with incorrectly-pitched instruments.