I was about to start a new topic with the same title but then I discovered this. 
Since the root cause was not resolved here decided to continue here.
I had the same problem building my snap, starting multipass, on Solus. When comparing the iptables setup with my Manjaro system that runs multipass just fine I noticed the following differences.
Manjaro
$ sudo iptables -S -P INPUT ACCEPT -P FORWARD ACCEPT -P OUTPUT ACCEPT -A INPUT -i mpqemubr0 -p tcp -m tcp --dport 53 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A INPUT -i mpqemubr0 -p udp -m udp --dport 53 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A INPUT -i mpqemubr0 -p udp -m udp --dport 67 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A FORWARD -i mpqemubr0 -o mpqemubr0 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A FORWARD -s 10.94.23.0/24 -i mpqemubr0 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A FORWARD -d 10.94.23.0/24 -o mpqemubr0 -m conntrack --ctstate RELATED,ESTABLISHED -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A FORWARD -i mpqemubr0 -m comment --comment "generated for Multipass network mpqemubr0" -j REJECT --reject-with icmp-port-unreachable -A FORWARD -o mpqemubr0 -m comment --comment "generated for Multipass network mpqemubr0" -j REJECT --reject-with icmp-port-unreachable -A OUTPUT -o mpqemubr0 -p tcp -m tcp --sport 53 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A OUTPUT -o mpqemubr0 -p udp -m udp --sport 53 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A OUTPUT -o mpqemubr0 -p udp -m udp --sport 67 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT
Solus
$ sudo iptables -S -P INPUT ACCEPT -P FORWARD ACCEPT -P OUTPUT ACCEPT -A INPUT -i mpqemubr0 -p tcp -m tcp --dport 53 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A INPUT -i mpqemubr0 -p udp -m udp --dport 53 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A INPUT -i mpqemubr0 -p udp -m udp --dport 67 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A OUTPUT -o mpqemubr0 -p tcp -m tcp --sport 53 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A OUTPUT -o mpqemubr0 -p udp -m udp --sport 53 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT -A OUTPUT -o mpqemubr0 -p udp -m udp --sport 67 -m comment --comment "generated for Multipass network mpqemubr0" -j ACCEPT
Not sure what to do with that knowledge though.