In Visual Studio 2017 I created a Console App (.NET Framework) project as a base and didn't change anything after creating the project before using the Export Template Wizard. I gave it a name and description, exported it, and then imported it into Visual Studio. When I go to create a new project, I can see my new template. It is named correctly, but the description just shows "MyTemplate" instead of the actual description I gave it.
I've made sure to follow the format shown on the Description element documentation page. It doesn't mention anything about a max length or forbidden characters. (Though even if it did, my description doesn't contain any special characters; just letters, spaces, and periods.)
Here is the portion of the .vstemplate file that was generated that includes my description (I haven't edited any part of the file):
...
<TemplateData>
<Name>MyNewTemplate</Name>
<Description>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque vitae mi vitae nibh convallis sagittis pretium id nisi. In rutrum enim quis ipsum gravida egestas. Donec facilisis sodales volutpat. Cras quis vulputate tortor. Vestibulum eget lectus augue. Fusce congue venenatis urna, ut fringilla mi scelerisque et. Nulla facilisi. Vestibulum ullamcorper facilisis quam id pellentesque. </Description>
<ProjectType>CSharp</ProjectType>
...
I've created other templates before and their descriptions have appeared just fine. This one doesn't, however.
After some experimentation, I found out the description appears when it contains 259 or fewer characters. So when I shorten it to the following, it works:
...
<TemplateData>
<Name>MyNewTemplate</Name>
<Description>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque vitae mi vitae nibh convallis sagittis pretium id nisi. In rutrum enim quis ipsum gravida egestas. Donec facilisis sodales volutpat. Cras quis vulputate tortor. Vestibulum eget lector entra</Description>
<ProjectType>CSharp</ProjectType>
...
The documentation doesn't say anything about a max length, and the template wizard doesn't prevent you from entering more than 259 characters. So if you find that your own template descriptions aren't appearing, check the length.