A licensee has reported a problem where FVisualStudioSourceCodeAccessor.AddSourceFiles is unable to handle two files being added at the same time. In this case, a .cpp and .h are added at the same time when a new class is created. Since FVisualStudioSourceCodeAccessor.AddSourceFiles has no way of handling a case where a "p4 add" fails due to it already being in the process of adding a file, the .cpp file is not added.
Edit: After speaking with the licensee further, this bug also causes the .cpp file to not be added to the Visual Studio solution as well.
This also occurred in 4.17.2
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-53121 in the post.