If the router is making requests, then the DNS for the Pi-Hole is known to the router. This information was likely passed to clients during lease renewal.
Since routers typically don't make requests on their own, the traffic from the router is actually from clients. It is likely that a configuration change resulted in clients showing on the Pi-Hole if they did not previously.
Please generate and upload a debug log and post the token here, so we can take a look at your configuration.
I'm pretty sure the requests from the router are generated by clients. In the case of my macbook (.254), for sure I make a lot more requests than what is showing in pihole (.17). My configuration is simple: The router is the DNS server for all clients that connect to it. Pihole (with dnscrypt) is the only DNS server for the router. I can't really control all the clients. Some of them could make requests using other DNS servers. But I find it odd that they would use directly the pihole since I never explicitly told any of them that there was another DNS server on the LAN.
It looks like the router is informing clients who is the actual DNS server (?!?!), but most of the time the network behaves as I would expect....