Groovy collections are not fully supported by ObjectDB (and by JPA in general). See the list of supported collections in JPA / ObjectDB on this ObjectDB Manual page.
If ObservableList works well, except the listeners, you may use JPA lifecycle events to clear listeners before persisting or updating the entity object (and then set them back if necessary).
Alternatively you can keep 2 list fields in your entity class. An ordinary List that will be persisted, and an ObservableList wrapper of that list that will be set as transient.