Thats not a true static IP but a static DHCP reservation instead.
The D in DHCP for dynamic.
What do you think happens with that DHCP reservation when you disable the DHCP service on the router to let Pi-hole take over?
If the Pi is to function as a DHCP server, it needs a true static IP configured manually on the Pi itself.
A DHCP server cant assign a DHCP lease to its own interface.
Check if below one is enabled:
systemctl is-enabled NetworkManager.service
If enabled, you can configure "manual" static details on the Pi with below (Network Manager Text User Interface):
sudo nmtui
Example config below (dont mind the lo profile for the loopback interface named lo):