Your debug log shows that you indeed blocked google.be exactly, not www.google.be:
*** [ DIAGNOSING ]: Domainlist (0/1 = exact white-/blacklist, 2/3 = regex white-/blacklist)
id type enabled group_ids domain date_added date_modified
----- ---- ------- ---------- ---------- ------------------- -------------------
(…)
11 1 1 0 google.be 2025-03-22 16:59:59 2025-03-22 16:59:59
google.be is neither www.google.be nor www.google.com.
In addition, your debug log shows that your router is distributing its own IP as DNS server via DHCP:
*** [ DIAGNOSING ]: Discovering active DHCP servers (takes 6 seconds)
Scanning all your interfaces for DHCP servers and IPv6 routers
* Received 300 bytes from 192.168.0.1 @ eth0
Offered IP address: 192.168.0.190
DHCP options:
Message type: DHCPOFFER (2)
dns-server: 192.168.0.1
router: 192.168.0.1
--- end of options ---
Clients will talk to your router for DNS, and your router then forwards those queries that it cannot answer itself to Pi-hole.
While Pi-hole filters your network's DNS traffic in that constellation, PI-hole would see all DNS queries as originating from your router, making your router Pi-hole's single client.
If you want to apply client-specific filtering, your clients have to talk directly to Pi-hole for DNS.
Instead of configuring your router to use Pi-hole as upstream, you should configure it to distribute your Pi-hole machine's IP as local DNS server via DHCP.