Unlike in tracker modules, the MIDI pitch wheel isn't unbounded but always has a fixed note range in which it can operate, and this note range has to be configured upfront. By default, OpenMPT uses a range of +/-12 semitones for sample-based instruments because
1) it's sufficient in most cases and
2) while the theoretical maximum resolution of the pitch wheel is 14386 discrete values (14 bit), a number of synths only supports 7 bit pitch wheel events with 128 discrete values. At +/-12 semitones pitch wheel depth, that's just ~5 pitch wheel steps per semitone.
Hence the default is a tradeoff between flexibility and accuracy.
For plugin-based instruments, OpenMPT uses the pitch wheel depth as configured in the instrument tab. So one possible workaround to increase the pitch wheel depth for a single sample-based instrument is to temporarily turn it into a plugin-based instrument and then set up its pitch wheel depth as desired.