I have tried this scenario by upgrading the library in my sample project using Microsoft.NETCore.App (2.0) to the latest version Aspose.Cells 19.7 but observed no issue. The library is upgraded successfully and sample code executes fine. Please note that after the release of Aspose.Cells for .Net Standard 19.2, a dependency is required for System.Drawing.Common and System.Text.Encoding.CodePages as mentioned in the Dependencies section here. Please ensure that while upgrading the library through NuGet package manager, these dependencies are also installed. If these are not installed, try to install them manually from NuGet package manager.
If still issue is there, please provide us with a newly created sample console project (runnable) with Aspose.Cells code only. We will try to reproduce the issue here and provide our feedback. Also, share the environment details with us.