CTE with HierarchyID suddenly causes parse error

Question

So I have this self-referencing table in my database named Nodes, used for storing the tree structure of an organization:

[Id] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](max) NULL,
[ParentId] [int] NULL,
(+ other metadata columns)

And from it I'm using HIERARCHYID to manage queries based on access levels and such. I wrote a table-valued function for this, tvf_OrgNodes, a long time ago, tested and working on SQL Server 2008 through 2014 and it's remained unchanged since then since it's been doing great. Now, however, something has changed because the parsing of HIERARCHYIDs from path nvarchars ("/2/10/8/") results in the following error, matching only 4 hits (!) on Google:

Msg 6522, Level 16, State 2, Line 26
A .NET Framework error occurred during execution of user-defined routine or aggregate "hierarchyid": 
Microsoft.SqlServer.Types.HierarchyIdException: 24000: SqlHierarchyId operation failed because HierarchyId object was constructed from an invalid binary string.

When altering the function to only return NVARCHAR instead of actual HIERARCHYID's, the paths all look fine, beginning with / for the root, followed by /2/ etc etc. Simply selecting HIERARCHYID::Parse('path') also works fine. I actually got the function working by leaving the paths as strings all the way until the INSERT into the function result, parsing the paths there. But alas, I get the same error when I then try and insert the reusulting data into a table of same schema.

So the question is, Is this a bug, or does anybody know of any (new?) pitfalls in working with HIERARCHYIDs<->Path strings that could cause this? I don't get where the whole binary string idea comes from.

This is the code of the TVF:

CREATE FUNCTION [dbo].[tvf_OrgNodes] () 
RETURNS @OrgNodes TABLE (
    OrgNode HIERARCHYID, 
    NodeId INT,
    OrgLevel INT,
    ParentNodeId INT
) AS 
BEGIN       
    WITH orgTree(OrgNode, NodeId, OrgLevel, ParentNodeId) AS (
        -- Anchor expression = root node
        SELECT    
            CAST(HIERARCHYID::GetRoot() AS varchar(180)) 
            , n.Id                            
            , 0                               
            , NULL                            
        FROM Nodes n         
        WHERE ParentId IS NULL -- Top level

        UNION ALL

        -- Recursive expression = organization tree
        SELECT
            CAST(orgTree.OrgNode + CAST(n.Id AS VARCHAR(180)) + N'/' AS VARCHAR(180))     
            , n.Id                           
            , orgTree.OrgLevel + 1           
            , n.ParentId
        FROM Nodes AS n
        JOIN orgTree 
           ON n.ParentId = orgTree.NodeId
    )
    INSERT INTO @OrgNodes
    SELECT 
        HIERARCHYID::Parse(OrgNode),
        NodeId,
        OrgLevel,
        ParentNodeId
    FROM orgTree;
    RETURN;
END

I might have recently installed .NET 4.53 aka 4.6 for the lolz. Can't find much proof of it anywhere except in the reg, though: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft.NETFramework\v4.0.30319\SKUs.NETFramework,Version=v4.5.3