All ZynAddSubFX presets retaining qualities if a non-default preset is previewed.

(Issue also on forums [here](https://lmms.io/forum/viewtopic.php?f=7&t=34452))

### Bug Summary
I downloaded a preset from the internet and previewed it in the sidebar by clicking. After this, all other (default) presets had changed sound characteristics.
Video explanation (easier to understand):
https://drive.google.com/file/d/1Z_Szg8jPrchu7wT07AUtE2V9rY_DxADv/view
This video is from when I first filed the report about a month ago.
I have placed another video lower in the post.

Note: this happened with **only two** of my three non-default presets, and incidentally both had a stereo enhancer on them and were very similar (one was an equalized version of the other).
I downloaded the original preset [here](https://lmms.io/lsp/?action=show&file=11157)

#### Steps to reproduce
1. Download above preset and place in presets directory
2. Open sidebar.
3. First check a default preset for sound behaviour, eg Bass 3 (not necessary, but helps making telling the difference before and after easier)
4. Preview downloaded preset by clicking and holding.
5. Now click any other preset, and it has changed. Ideally, click the preset you clicked earlier (eg, if you clicked Bass 3 in Step 3, click Bass 3 again and you can tell the difference.)

#### Expected behavior
Previewing this preset shouldn't change characteristics of other presets to be similar to it.

#### Actual behavior
Previewing this preset [changes qualities of other presets to sound like it](https://drive.google.com/file/d/1Z_Szg8jPrchu7wT07AUtE2V9rY_DxADv/view)

#### Screenshot
(Only video, no screenshot)

#### Affected LMMS versions
1.2.0, 1.3 alpha, presumably everything in between.

Note that the bug happens only with this preset, not any other. I have three presets in my folder: "Super Metal Bass Guitar"; "SMBG" (which is essentially the same thing but equalized slightly); and "Weird synth thing". The first two presets only give this problem, and both have (disabled) stereo enhancers. The stereo enhancers are not causing the issue.
Also, clicking the first two changes the sound of all default presets, but clicking "Weird synth thing" resets them to normal.
See:
https://user-images.githubusercontent.com/76674645/124590875-42ea6a00-de79-11eb-8eee-b7732d61d118.mp4

Start by reproducing the issue from the sidebar with the linked preset, then trace the preset-preview path and ZynAddSubFX preset loading. Compare the instrument state before and after previewing the preset, including the disabled stereo enhancer. Done means previewing it no longer changes the characteristics of default presets.

Written by the indexing model from the issue text.