I think this might be related to Widespread SERVFAIL problem related to DNS 0x20 / https://lists.dns-oarc.net/pipermail/dns-operations/2019-January/018359.html .
Specifically, the h-dns.pl nameserver is affected within the .pl ccTLD
I was finally able to reproduce the SERVFAIL issue with Unbound 1.9.0rc1 against your domain (log attached).
unbound-pl-fail.txt (41.5 KB)
I am unsure if you can actually do anything about it though, @mnordhoff might be able to confirm.