You had also mentioned that this is a plug-in you're using. Is this a third-party plugin you that installed to GarageBand or is plug-in already included with GarageBand? If this is a third-party plug-in that you use, you might have to reinstall that plug-in to see if it helps address this issue. If this plug-in was included in GarageBand itself, you might have to uninstall and re-install GarageBand to further isolate this issue. Before attempting this, please save any audio tracks that you're currently working on. Just in case, the resources below can help: