I have not been able to find an example of using Powershell to edit/create MultiString registry entries.
As an example, I’m working with automating configuration of Tomcat, running as a windows service (ick).
It appears to create it correctly, however when I open the registry key in RegEdit, the contents display as one continuous string, instead of each property being on a different line.
In your example, $a is a one-element array containing a string that happens to have embedded newline / carriage return characters. What you actually need to send is a multi-element array of strings, like this: