from https://redux-toolkit.js.org/rtk-query/api/created-api/hooks#usemutation

fulfilledTimestamp?: number // Timestamp for when the mutation was completed

should have a capitalized 'S' as fulfilledTimeStamp

For the record since timestamp is a compound word I would not have expected the S to be capitalized however this project seems to have used that as a convention. So the doc will match the API at least.