Very frustrating, MS is not supporting this product at all. We have no response here from MS for years. I always have to fix bugs and patch things. I think this product has to be one of their worst.
@FrancisL and anyone else here ... I built a very simple solution using the HTTPS action.
This should get what you need, feel free to replace GetById with GetByName and use the list name (I just know that I change mine a lot😞
Create a Send an HTTP request to SharePoint action as a POST in your flow:
URI - _api/web/lists/GetById('YOUR_LIST_ID')/items(ITEM_ID)
Headers - code follows list - You need Content-Type, Accept, IF-MATCH, and X-HTTP-Method, headers.
Body - code follows list - Clarify the '__metadata' 'type' to "SP.Data.LIST_NAMEListItem" (where LIST_NAME is case sensitive, with a capitalised first letter), then list any fields (case sensitive) that you'd like to make null, and set the value to null.