Can't change preset name for AU instruments in a patch

Hi, I'm using Mainstage 3.2.4, and I'm trying to change the parameter "preset name" for AU instruments in a channel strip alias, changing a patch.

I can do this for Mainstage builtin effects, for example, if I create a channel strip with "Space Designer" and I use it as alias in two patches, I can create a button control mapped to the parameter "preset name" of "Space Designer", and in the "saved value" parameter I can set the preset that I want to load in that patch.

So when I load patch 1, Space Designer load one preset, and when I load patch 2, Space Designer load another preset.

I would like to do the same for AU instruments, but the "saved value" parameter is disabled and empty, and I can't select a preset.

I've tried with Kontakt Player, and the builtin AU instruments in Mainstage, with the same result.

If I open the AU instrument, I can see the presets that I've created. The presets are saved as .aupreset files in the folders "/UserName/Library/Audio/Presets" and in "/Application Support/Logic/Plug-In Settings/PluginName"