I have this build.psd1 file:

@{
    ModuleManifest = "ModuleName.psd1"
    CopyDirectories = @("Formats", "Lib")
    Target = "CleanBuild"
    Prefix = "./Prefix.ps1"
    Suffix = "./Suffix.ps1"
}

When I run Build-Module .\build.psd1 the contents of the prefix file are not copied in to the resulting psm1 file, but the suffix is included.

I verified that this works if I load the v1.0.0 module.