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I had the same issue, and after two days of debugging finally found a workaround.

• There is a Grails utility class called org.grails.datastore.mapping.reflect.FieldEntityAccess, apparently used to access entities fields using reflection. That class has an inner class called FieldEntityReflector. Every time a FieldEntityAccess instance is created, there is an instance of FieldEntityReflector created internally. All those instances are stored in a static map called REFLECTORS, whose keys are the entity names, apparently for performance reasons.
• During bean initialization, mongoDatastore bean creates one FieldEntityAccess for each entity, passing a DocumentPersistentEntity as constructor parameter.
• In the same way, grailsDomainClassMappingContext bean, unaware of mongoDatastore creates its own FieldEntityAccess for each entity, passing down KeyValuePersistentEntity as parameter. Those FieldEntityAccess instances and their corresponding EntityReflectors are unaware of mongo and therefore don't support mongo specific types like Point.
• Given that the REFLECTORS map keys are entity names, EntityReflectors created last for the same entity, override previously created reflectors, and are used by all EntityReflector instances.
• When grailsDomainClassMappingContext reflectors are created last, they are used when mongo tries to persist and entity and therefore throw an error.

In development mode, UrlMappingsGrailsPlugin plugin creates some beans used to allow url mappings reloading. Those beans trigger instantiation of grailsDomainClassMappingContext bean before mongoDatastore bean. As the mongo beans are created later, the error is not thrown. But, in production mode, those beans are not created, and mongoDatastore is created before grailsDomainClassMappingContext, therefore throwing the error.

1. Force grailsDomainClassMappingContext bean instantiation before mongoDatastore: I was unable to do that after several attempts, maybe because of grails bean instantiation logic.
2. Force UrlMappingsGrailsPlugin instantiation even on production mode: I prefered not to go that way due to possible permformance implications.
3. Use the static method clearReflectors of FieldEntityAccess class to clear all the reflectors after system initialization:** I decided to do that and the system started to work. I haven't tested it in depth, but was able to persist Point correctly.

In order to call the method, I created a bean depending on both grailsDomainClassMappingContext and mongoDatastore and called FieldEntityAccess.clearReflectors()