DNS sometimes not working

I’m using a DYNv6 URL to address my Fritz!Box.
Sometimes a “ping” to this address works as expected and gives the internet-IP-address of the router, but sometimes ping gives “ping request cannot find host…”.
If I tried to ping to the URL 2 - 9 times - someday it works!

But: a “ping” directly to the IP is always working!

What could be the reason for that??


I realized, that yesterday in the afternoon the URL dynv6.com was totally unavailable.
Now the site is available and the pings seems to work now.
Are these incidents related?