I found this solution. The errors have disappeared and it would seem to work. I keep testing.
For all Transformable attributes, I have set “Transformer” to “NSSecureUnarchiveFromData” in the Data Model Inspector panel.
EDIT:
After a few days of testing I add something to my previous solution.
The previous solution works if, after the changes, the application is deleted. Otherwise, the data model is not recognized and is created from scratch, losing all the historical information (and in production this is not acceptable!!!).
The final solution I adopted was to add these changes to a new model (following this link) and implementing the migration (always described in the link).
In this case the warning appears only once after the update and then disappears.