Why will DHCP break if you have the server's switch port tagged?

So, I know this doesn't work because it always happens, but why?

Example:

I have a default VLAN on a switch and its DHCP server is on, for example, port 3 giving 192 addresses.
Port 1 is a trunking port to firewall with VLAN 1 and 10 tagged.
I tag all 24 ports on the switch with 10, untagPVIDonly.
I plug a phone into the switch and manually assign VLAN 10.
Phone is set to VLAN 10 but gets the default VLAN's DHCP of 192.
I take VLAN 10 off of the DHCP servers switchport 3, making it an access port on the native VLAN, and everything works. The phone now get's it's 172 address from the firewall.
Why does this happen? The most I can work out is that the broadcast for DHCP on port 3 is somehow bleeding into VLAN 10.