When using the Update Item action for a SharePoint List, it is not currently possible to clear a value that has been previously set in a Person or Group type field (Claims). Attempting to do this currently causes the Flow to fail, with a "Badgateway" 402 error, arising from a "user does not exist" 400 error.
Please enable the dynamic content expression to allow use of the Null() function to clear previous content from ANY field type, including person or group claims fields.