strange behavior DHCP traffic allowed during reboot

I have a EdgeRouter X SFPv1.6.7

I have eth1 internet - provider router with DHCP 

                             or

                             bridge mode - cable modem 

The "or" above does not influence behaviour 

eth0 lan 2

switch0 lan 1 

eth2,3,4,5

Edge router DHCP eth0 class 192.168.200.0/24

Edge router DHCP switch0 class 192.168.300.0/24 

external wifi connected to eth0

no dhcp on wifi 

2 PCs connected to wifi both with IPs from 192.168.200.0/24

PC 1 issues reboot to edge router - THIS ONLY happens during reboot at boot UP 

PC2 does a DHCP release and DHCP renew 

surprise PC2 get an IP for the eth1 connected provider DHCP or bridge 

There is no doubt I have tested by removing the cables.

is there any way to block this ? I have added a rule on eth1 to deny all DHCP requests to outside but seems not to work...at boot time

This does not happen when router is up and I simply disable the DHCP on eth0 in order to test.only when edge reboots

Behaviour is the same with 1.9.0 image