[Bug] Mixer chanels muted after solo-ing one after saving

### System Information

Windows 10 - Intel(R) Core(TM) i5-8265U CPU @ 1.60GHz 1.80 GHz - 8GB RAM

### LMMS Version(s)

LMMS 1.3.0-alpha.1.833+e50f312 (MinGW)

### Most Recent Working Version

_No response_

### Bug Summary

After saving a project with a mixer channel in solo, some remain muted even when un-solo-ing that channel.

### Expected Behaviour

- I put a mixer channel in solo
- Saved the project, then loaded it
- After un-solo-ing the channel, a lot of channels remained muted, with the only option to manually click the Mute button once for them to behave normally again.

### Steps To Reproduce

https://github.com/user-attachments/assets/03e7ab12-2b7f-4145-9938-3350dd5eab35

### Logs

Click to expand

### Screenshots / Minimum Reproducible Project

I made a short demo where you can reproduce it:
[mute bug demo.zip](https://github.com/user-attachments/files/20854457/mute.bug.demo.zip)

### 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 opening the attached mute bug demo in LMMS 1.3.0-alpha.1.833+e50f312 and reproduce the save, reload, and unsolo sequence. Trace how mixer-channel solo and mute state is preserved across saving and loading; done means unsoloing after reload restores every affected channel without manual mute toggles.

Written by the indexing model from the issue text.