In the 4.17 release notes, there is a note that mentions that plugins added to a directory in the AdditionalPluginDirectories list in the .uproject file would automatically get included in the generated code project for Visual Studio. This does not appear to actually be happening.
The code for the plugin is not included in the solution.
The code for the plugin is included in the solution.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-51972 in the post.