Incorrect mapping of MIDI controllers to mixer faders

### System Information

Windows 11, M-Audio Oxygen 61 MKV MIDI controller

### LMMS Version(s)

1.3.0-alpha.1.884+a809c03

### Most Recent Working Version

1.2.2

### Bug Summary

When mapping a MIDI controller to a mixer fader, movements of the controller aren't translated correctly, with spikes of movement in lower values, 0.0 dB being in the middle and little to no movement above 0.0 dB. This only happens with mixer faders, too. Parameters in other locations translate incoming MIDI values correctly. Below are two videos attached showing this behavior.

### Expected Behaviour

MIDI values should be correctly translated to mixer volume without jitter/sudden jumps.

### Steps To Reproduce

1. Have a MIDI controller
2. Create a new LMMS project
3. Either use the master channel or add a new mixer channel
4. Map the MIDI controller to a mixer channel
5. Try to adjust the volume of the channel

### Logs

Click to expand

### Screenshots / Minimum Reproducible Project

- Video showcasing the issue: [https://imgur.com/a/Qf8Dgm4](https://imgur.com/a/Qf8Dgm4)
- Video showing a non-mixer fader working correctly: [https://imgur.com/a/TYCUJ9w](https://imgur.com/a/TYCUJ9w)

### Please search the issue tracker for existing bug reports before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by reproducing the mapping described in the five steps on Windows 11 with an M-Audio Oxygen 61 MKV, using the two linked videos as behavioral references. Compare mixer fader response with a non-mixer parameter, then verify that MIDI values map smoothly across the full volume range without jitter or sudden jumps; no source file or test is identified in the report.

Written by the indexing model from the issue text.