Issue Looking up ocsp.int-x3.letsencrypt.org more than 5 times a minute ?

[Heinrich]

Hi,
I rund PLESK Obsidian on ubunut 16.04. I have the extension for letsenctypt installed and I recently started to use an DNS-provider named nextdns.io.

This one is free for less then 30.000 DNS requests per month, however, it's analytics page shows that my server looks up ocsp.int-x3.letsencrypt.org mor than 13.000.000 times in that time frame.
Any ideas why?
Shouldn't DNS requests be cached locally?