IoT network not DHCPing anymore

I think you ultimately performed the steps outline in the release notes at the bottom of the Known Issues.

Based on the limited info in the Known Issue, I'm assuming there is some race condition taking place where the DHCP service or iptables rules are set before the vlan interface has been created. Enabling telnet access through the debug page can help you recover if you have some linux knowledge. Personally though, I have up on DNS and DHCP on my SX(R/S)80 and instead have a raspberry pi connected via a trunk port handing out addresses on all the VLANs. The main reason was to be able to have DNS for all DHCP hosts.

I have not experienced this issue though so maybe it's just a buggy DHCP server/config? I attached my dnsmasq config and ubuntu core netplan config incase anyone else wants to try and see if it helps.

dnsmasq.conf

domain-needed
expand-hosts
localise-queries
bogus-priv
no-resolv
server=/test/
server=/localhost/
server=/invalid/
bind-dynamic
cache-size=10000
conf-dir=/etc/dnsmasq.d/,*.conf
domain=lan
dhcp-authoritative
dhcp-leasefile=/etc/dnsmasq.d/dhcp.leases
localise-queries
dhcp-range=192.168.1.100,192.168.1.199,8h
dhcp-option=3,192.168.1.1
dhcp-range=eth0.20,192.168.20.100,192.168.20.199,8h
dhcp-option=eth0.20,3,192.168.20.1
dhcp-range=eth0.30,192.168.30.100,192.168.30.199,8h
dhcp-option=eth0.30,3,192.168.30.1
dhcp-range=eth0.40,192.168.40.100,192.168.40.199,8h
dhcp-option=eth0.40,3,192.168.40.1

netplan.yaml

network:
    ethernets:
        eth0:
            addresses:
            - 192.168.1.2/24
            gateway4: 192.168.1.1
            nameservers:
                addresses:
                - 127.0.0.1
    version: 2
    vlans:
        eth0.20:
            addresses:
            - 192.168.20.2/24
            id: 20
            link: eth0
        eth0.30:
            addresses:
            - 192.168.30.2/24
            id: 30
            link: eth0
        eth0.40:
            addresses:
            - 192.168.40.2/24
            id: 40
            link: eth0