Assuming you are issueing static leases via Pi-hole's web UI:
It seems that Pi-hole stores information on static leases in
/etc/dnsmasq.d/04-pihole-static-dhcp.conf
This configuration file employs the dhcp= option as well, and values defined there seem to take precedence over your own settings.
You can manipulate the entries in 04-pihole-static-dhcp.conf directly, which will give the result you desire, but I advise CAUTION.
You'll likely be stuck with editing static leases manually ever after, since the Pi-hole admin panel doesn't agree well with this:
I suspect that adding another static lease using Settings|DHCP panel could render some DHCP leases unusable, but haven't tried that.
I wouldn't recommend this as a general solution, but only if you feel comfortable with it and if you're familiar with dnsmasq, it's probably worth a try.
Keep in mind that this file is under Pi-hole's control, so can be affected at any time to Pi-holes liking, maybe so during updates.
As I am just a Pi-hole user, I'd recommend you should seek confirmation by a moderator that indeed there is no better way to reach your goal before you permanently employ this workaround.