Isn’t this the same issue you had here?
Setting a field in extensible storage removes the other fields from the entity - Developers - Dynamo (dynamobim.com)
Extensible storage is not a data management architecture. It doesn’t work like parameters or properties or dictionaries or anything else you might be used to. It’s just “dummy storage”. Whatever you write to it is what gets stored. If you want to add or modify something you need to write the full “block” of data to storage. The data itself can use hierarchies and relationships in whatever format you like, but the entity itself is just a copy of that data. It does no retain its relationship with the element.