Creating a plugin with the use of script context that is not set to Legacy will not run the script. The rbxmx contains 3 Scripts, Legacy, Client, Server, the code for them are:
What you’re seeing here is intended behavior. We actually have an additional option for RunContext that is not currently implemented called Plugin which will be the eventual way to do this.
Legacy will also work in plugins for backwards compatibility reasons and is the current way to create plugin scripts.