ADO .NET Entity Framework edmx losing the connection string

Question

I'm using VisualStudio 2019 as IDE. I used ADO .NET Entity Framework for MySQL 5.6. The solution is arranged in several project.

I moved MySQL database from 5.6 to 8.0.31.

To update the edmx model, I deleted the edmx file created with MySQL 5.6 and created again using the MySQL Connector for Visual Studio for MySQL 8.0.31.

I was able to create the edmx file. The point is that every time I close the edmx file, if I open again, in the property panel the connection string value disappears.

This causes that I cannot update the model from the database as the connection string is not valid.

However if I

1. open the App.config file
2. remove the connection String
3. paste exactly the same string value
4. save the App.config file

And I open again the edmx file, the connection string parameter is set!!!

Why does this happen and how to fix this behaviour?