I am seeing the same issue with Powershell for WEEKLY occurrence
Object Being Loaded :
Recurance FREQ=WEEKLY;INTERVAL=1;BYDAY=SU
DueDateTimeTimeZone
Assignee Brian Kendrick
ListName Household chores
Name Blow porches, driveway, and side deck
ListID taskList/8929837_12084023
CreationDate 2023-05-28T10:51:44.000Z
Catagory Regular Maintenance
DueDate 2023-05-28T13:00:00.000Z
Body Parameters
Categories {Regular Maintenance}
Title Blow porches, driveway, and side deck
CreatedDateTime 2023-05-28T10:51:44.000Z
Recurrence {firstDayOfWeek, interval, type, daysOfWeek}
DueDateTime 2023-05-28T13:00:00.000Z
Recurrence Pattern
firstDayOfWeek sunday
interval 1
type WEEKLY
daysOfWeek ["sunday"]
Error Response
New-MgUserTodoListTask : The property 'firstDayOfWeek' does not exist on type 'Microsoft.Graph.PatternedRecurrence'. Make sure to only use property names that are defined by the type.
...
+ CategoryInfo : InvalidOperation: ({ UserId = c2d6...GraphTodoTask }:<>f__AnonymousType71`3) [New-MgUserTodoListTas k_Create1], RestException`1 + FullyQualifiedErrorId : invalidRequest,Microsoft.Graph.PowerShell.Cmdlets.NewMgUserTodoListTask_Create1