Alright, i found the issue. To avoid the problem I had to add a backlink to the ToMany property in order to make it work properly :

@Entity()
class MyObject {

@Backlink('holderObject') // This was missing
final insideObjects = ToMany<InsideObject>();

MyObject({
 this.name,
});

...
}

Here the foc info from objectbox :

When using @Backlink it is recommended to explicitly specify the linked to relation using to. It is possible to omit this if there is only one matching relation. However, it helps with code readability and avoids a compile-time error if at any point another matching relation is added (in the above case, if another ToOne is added to the Order class).