Most likely scenario is that it doesn’t copy the files into the build output - you can try to do a dotnet publish locally through the cli and see if the app plugin files are there.
If they are not then you can force it by adding this to your website csproj:
<ItemGroup>
<Content Include="App_Plugins\MarketingDashboard\**\*.*">
<ExcludeFromSingleFile>true</ExcludeFromSingleFile>
<CopyToPublishDirectory>PreserveNewest</CopyToPublishDirectory>
</Content>
</ItemGroup>