Using TypeSet in provider always adds an empty element on update

I’m writing a custom provider and am facing an issue with TypeSet.
Maybe it comes from a misunderstanding on my side but i can’t explain why it behaves this way.

I have a TypeSet in my schema with a resource element.
Whatever i do, when terraform calculates a plan and calls the resource update function, i always receive a set with items that are initialized with default values for the underlying schema.

For instance, i have 6 elements in the set and i want to keep only 1. Terraform first reads the resource to refresh the state, the read returns the 6 elements, then it calls update with a set containing 2 elements (the item coming from the config, plus another item with everything initialized with default value).

I would expect that update receives a set containing only 1 element.

Am i missing something ? Is it how a TypeSet should behave ?

This doesn’t quite sound like normal behaviour, but we’ll need more info to understand what’s going on here. Could you link to the code in question, or post at least the schema, update function, and config?