Timeout when CASE statement matches

[thefreakquency]

While trying to pimp the solution to https://thwack.solarwinds.com/product-forums/the-orion-platform/f/orion-sdk/94568/device-temperature, I ran into an issue.

When running the following query, if there are no match to the first CASE statement, all goes well:

SELECT TOP 100 N.Caption AS [Device name],
    N.IP_Address,
    N.MachineType,
    N.Vendor,
    N.HardwareHealthInfos.ServiceTag AS [Serial Number],
    T.AVG_Node_Temp_Celcius AS [AVG Temp in C_plain],
    T.AVG_Node_Temp_Farenheit AS [AVG Temp in F_plain],
    CASE 
        WHEN T.AVG_Node_Temp_Celcius > 50
            THEN CONCAT (
                    '<font color="red"><b>',
                    T.AVG_Node_Temp_Celcius,
                    '</font></b>'
                    )
        ELSE T.AVG_Node_Temp_Celcius
        END AS [AVG Temp in C_html]
FROM Orion.Nodes AS N
LEFT OUTER JOIN (
    SELECT NodeID,
        CASE 
            WHEN H.HardwareUnit.Name = 'DegreesC'
                THEN ROUND(AVG(H.Value), 0)
            WHEN H.HardwareUnit.Name = 'DegreesF'
                THEN ROUND(((AVG(H.Value) - 32) * 5 / 9), 0)
            END AS [AVG_Node_Temp_Celcius],
        CASE 
            WHEN H.HardwareUnit.Name = 'DegreesC'
                THEN ROUND(((AVG(H.Value) * 9) / 5 + 32), 0)
            WHEN H.HardwareUnit.Name = 'DegreesF'
                THEN ROUND(AVG(H.Value), 0)
            END AS [AVG_Node_Temp_Farenheit]
    FROM Orion.HardwareHealth.HardwareItem AS H
    WHERE H.HardwareUnit.Name IN (
            'DegreesF',
            'DegreesC'
            )
        AND H.IsDeleted = 'FALSE'
        AND H.Value IS NOT NULL
    GROUP BY H.NodeID,
        H.HardwareUnit.Name
    ) AS T ON N.Nodeid = T.Nodeid
WHERE N.Vendor = 'Cisco'

I receive 59 rows in my query in less than a second.

If I am lowering WHEN T.AVG_Node_Temp_Celcius > 50 to something like 30 in order to make sure it matches something in my environment, the query runs, and timeout after 2 minutes:

I know that the CONCAT statement in my case is fine. To verify it, I replaced the whole CASE statement by CONCAT ('<font color="red"><b>', T.AVG_Node_Temp_Celcius,'</font></b>') AS [AVG Temp in C_html] and I got the expected results:

Shouldnt a CONCAT statement in a CASE work?