Are you by any chance trying to set the field by directly accessing it from the DynamicContent object like a property?
myDynamicItem.RelatedAsset = someGuidArray;
This will not work for DynamicContent objects. You need to use GetValue() and SetValue() extension methods respectively when you want to work with DynamicContent fields.
You can check the following Documentation article:
Note to add Telerik.Sitefinity.Model namespace to your usings in order to be able to access those methods.
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