When upgrading a database it is a common procedure to duplicate the database's folder and to open it using the version you want to upgrade to. If you do that, open the project file using the upgraded version of 4D and attempt to recompile the database, the following alert is displayed:
This should come as a surprise since the database was upgraded. The reason why this is happening is that the project stores the absolute path to the database, so it is still pointing to the original database.
So, as a general rule, you should either consider upgrading the original database (not the copy) or simply create a new project.