I just moved primary dns and dhcp from 16.04.6 LTS to 20.04 LTS and also observe this behavior.
Primary is isc-dhcp-server 4.4.1-2.1ubuntu5 amd64. Crashes regularly.
Secondary is isc-dhcp-server 4.3.5-3+deb9 armhf which runs fine.
I just moved primary dns and dhcp from 16.04.6 LTS to 20.04 LTS and also observe this behavior.
Primary is isc-dhcp-server 4.4.1-2.1ubuntu5 amd64. Crashes regularly.
Secondary is isc-dhcp-server 4.3.5-3+deb9 armhf which runs fine.