Hi,
Probably that is happened because the entity from the database that is the original one is from other module and is with the property "Expose read only" at Yes.
If you change the value from the original database entity in the property "Expose read only" to No you will have the second option ( read/write ) (Don't forget to publish and refresh dependencies in the mobile module).
NOTE: Evaluate that condition because it's not a common good practice to have entities with the property "Expose read only" with value No.
Hope this can help.
Best regards,
Ricardo Pereira