The important word here is external.
Pi-hole is not blocking this domain, but the upstream DNS (9.9.9.9) is blocking it.
Here, Quad9 can't resolve the domain:
rdweb@server: ~ $ nslookup mia.nl.tab.digital 9.9.9.9
Server: 9.9.9.9
Address: 9.9.9.9#53
** server can't find mia.nl.tab.digital: NXDOMAIN