If you selected to install a version on the “Details” page of the plugin and then clicked also the “Install” button next to the “Details” button this seems to happen.
As I wrote in my previous post exclusively use one of them: Either on the “Details” page e.g. “Install v3.3.1(beta)” or the “Install” button next to the “Details” button. Then change to the page where you see the installed plugins and wait patiently until the Touch Display plugin appears.
I assume this is not related to Volumio3 RC1. I suggest we proceed with this issue in the Touch Display plugin thread or you open a new thread.
Edit: @Wheaten has reported that the latter problem is no longer present after a new installation of Volumio.