Can confirm. If I have a secondary DNS specified in my router, other than my pi-hole, it becomes useless. My guess is the router is either measuring response time, and goes with the most efficient, or otherwise round-robining the requests. Either way, requests bypass the pi-hole in such quantities that it became useless.

PS: someone here mentioned that this behavior is OS-dependent - nope, this happens on the router level, and all devices in my apartment suffer.