As the issue you are reporting is related to dnsmasq as a DHCP server, I’ve removed PiHole from the topic title which is purely dnsmasq as a DNS server.
Using false information in your report helps nobody, and creates unnecessary confusion.
The reason for the long post is just for helping you to get the BUG that is killing dnsmasq when is used as a DHCP server, and not just a DNS server as PiHole does.
If you report a bug, we need to know how you have configured the software, which in this case is dnsmasq as a DHCP server.
What is the contents of your /etc/dnsmasq.conf file?