Why is the instrument doing this?

[cursive lintel]

It also randomly changes the pitch

I think it has something to do with being midi?

[cursive lintel]

It's only happening with zynaddsubfx too

[limber pelican]

I think that is not likely, zyn is an inbuilt plugin.

[noble night]

That is because you have closed the LP-filter in the instrument

When you click on prehear, the raw instrument is played. When you click on the piano the modulated instrument is played. If you do not understand filtering and filter (LP, HP, Band-P Notch.. etc -i advise you to study the subject. It is very important in sound synthesis.
In my videos i have masterclass for zasfx where deep settings are explained https://www.youtube.com/playlist?list=PLMvza7ri1L0ci7GuQ0a0D_Fa4NWWj70pD

[cursive lintel]

How exactly do I revert it then?

Oop- I accidentally found it, changed it, and it works now

I didn't change it originally though

[limber pelican]

So you changed it but then you didn't change it? This is confusing.
It sounds like you are experiencing a bug. I would like to know how to replicate this. Would you mind sharing how to make this issue occur step by step?

[cursive lintel]

I meant I changed the setting, but I didn't originally change it to do that, I don't believe I changed any setting to do that

[limber pelican]

So is it a bug?

Somebody updated Zyn's low pass filter some time ago. This could be the reason why how Zyn works changed in your project. How old is the project?

[noble night]

Somebody updated Zyn's low pass filter some time ago

I sincerely hope nót

[limber pelican]

I'm not sure what was done to it so it might not be true

[cursive lintel]

I made the song in a single day

[limber pelican]

All right, I think I can not help with this issue because I do not know enough information.

[cursive lintel]

Thanks anyways