Org chart CTE with exact index but plan shows expensive sort node

I've got a moderately large table (148 million rows) that's got pretty common org chart attributes to it. Names are changed, but it's the same idea. I was thinking of adding a hierarchyid column to the mix; then I could index that and hopefully make some of the queries faster.

This query is modeled on the msdocs sample on hierarchy id. Before I ran it, I put indexes on (employeeid), (managerid, include employeeid), and (officeid, managerid, employeeid) hoping to make it as fast as possible. But the query took over 5 hours to run.

I looked at the plan, and the (officeid, managerid, employeeid) index is used for both parts of the cte.

What I don't understand is that the ROW_NUMBER() windowing function also appears to have added a SORT node, which ss says is taking 94% of the query time. It's already using an index that is pre-ordered in the same way, so why is there a SORT node taking up 94% of the time?

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Thanks

WITH paths(path, duns, extended_gu, extended_parent)   AS (  -- This section provides the value for the root of the hierarchy      SELECT hierarchyid::GetRoot() AS path, employeeid, officeid, managerid    FROM OrgChart where officeid = 0    -- This section provides values for all nodes except the root      UNION ALL       SELECT CAST(p.path.ToString() + CAST(ROW_NUMBER() OVER (PARTITION BY RD.officeid, RD.managerid     ORDER BY RD.officeid, RD.managerid) AS varchar(30)) +'/' AS hierarchyid),       RD.employeeid, RD.managerid, Rd.officeid    FROM OrgChart rd       JOIN paths AS p ON rd.managerid = P.employeeid) UPDATE rd set tree=P.PathFROM Paths PINNER JOIN  OrgChart rd on rd.duns = P.Duns

EDIT: Schema, added in response to comment below. I don't see a "Paste The Plan" as a feature in the editor; where is that? Rather than null on the roots, those ids are 0

CREATE TABLE [dbo].[OrgChart](    [employeeid] [int] NOT NULL,    [managerid] [int] NOT NULL,    [officeid] [int] NOT NULL,    ...    [tree] [hierarchyid] NULL) ON [PRIMARY]GOCREATE UNIQUE NONCLUSTERED INDEX [uixOrgChart_employeeids] ON [dbo].[OrgChart](    [employeeid] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]CREATE UNIQUE NONCLUSTERED INDEX [uixOrgChart_officemgremployee] ON [dbo].[OrgChart](    [officeid] ASC,    [managerid] ASC,    [employeeid] ASC)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]CREATE NONCLUSTERED INDEX [uixOrgChart_mgremp] ON [dbo].[OrgChart](    [managerid] ASC)INCLUDE([employeeid]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]