SELECT with lots of left join is very slow in SQL Server 2014

I've got a query with, more or less, this structure:

SELECT 
    MAINTABLE_ID
FROM 
    MAINTABLE WITH (NOLOCK)
LEFT JOIN
    TABLE_1 WITH (NOLOCK) ON TABLE_1_FK_MAINTABLE_ID = MAINTABLE_ID
LEFT JOIN
    TABLE_2 WITH (NOLOCK) ON TABLE_2_FK_MAINTABLE_ID = MAINTABLE_ID
[...]
LEFT JOIN
    TABLE_N WITH (NOLOCK) ON TABLE_N_FK_MAINTABLE_ID = MAINTABLE_ID   
WHERE 
    TABLE_1_ID IS NULL
    AND TABLE_2_ID IS NULL
    [...]
    AND TABLE_N_ID IS NULL      

N is about 50. The table names are aliases to four part named tables. This query is pretty fast from SQL Server 2008 to SQL Server 2012. But in SQL Server 2014 (same database) it never ends. The query has been tested with different hardware configurations (these work with the others editions of SQL Server).

What's wrong with it?

Thank you.