plugin wrapper bidirectional

If something doesn't work, monitoring what comes out of your objects usually helps you finding what doesn't work 98% of the time. You could do that simply by attaching the right inlet of a message to your objects output, or a [print], and/or enable Event Probe in the Debug menu and hover the patch cords to see the last message that transited in that patch cord.

Doing so in your patch would make you realize [zl.slice] left outlet won't output anything, because the object is missing an argument (the number or items to slice). Here, no argument is equivalent to [zl.slice 0], which means 0 item will get out from the left outlet and the remaining items (the entire list) will get out from the right outlet. So I guess what you want here is [zl.slice 1].

But this said, I would prefer to use the normalized float param value instead of the string one, since I assume scaling is more efficient than parsing a string to extract the value from it.

And because I started speaking about optimization, I would use one [route], or better, one [spray], since it is more efficient to have one object filtering and distributing many messages to many destinations than have multiple objects filtering the same amount of message but distributing them to only one destination each.

EDIT: Forgot to mention I added a [prepend set] to avoid feedback loops, although it doesn't seem to be necessary for this specific VST. But a a quick search on the forum showed others where it is required.