This is how Mendix works, the domain model is leading for the database. When using a existing database, you have to question the following;
Is the database used by other applications?
Note: the Mx database can be an oracle database, bt will by managed by the Mendix domain model
When a Mendix server starts, it's using the database credentials provided and first checks the existing ‘data model’ (all tables) present in the specified database schema.
It checks some Mendix system tables to see what changes are required to align the data model with the current application model (added/renamed/removed entities/attributes/associations). If it doesn't recognize the model, it will completely build the data model from scratch.
Did you fully exported/imported the database? Did you change the schema name?