long FQDN resolution

[DEMBLANS Mathieu]

Hello,
I have a question about mecanism for requests of long subdomains FQDN.
Our DNS, which is in recursive configuration, split long fqdn request with subdomains requests like :

Original request from the client to our recursive DNS : A a.b.c.d.example.com

Requests done by our DNS to the domain example.com NS:
A  _.c.d.example.com
A _b.c.d.example.com
A a.b.c.d.example.com

Is it a normal behaviour ?

This is only done for 4 levels domains (a.b.example.com for example) and more, not < normal > fqdn (ex : www.example.com).
What part of the configuration manage this mecanism ?
Is this really usefull ?

It is problematic for DNSBL requests because it generate a lot of useless requests and this kind of service look at the number of requests done (usage policy):

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