【Issue】
There is an VLAN issue within RouterOS above 7.12. IP address cannot be obtained through the DHCP client (with Option 12 and 60) . It is ok in RouterOS 7.11.3(tested)、7.8(tested).

【Network Topology】
The modem is connected to the RouterOS ether6-wan port through ONE network cable. IPTV VLAN 2501 is bound to it.

【RouterOS Configuration】
/interface bridge
add name=bridge1-lan
add admin-mac=14:17:2A:B4:DB:38 auto-mac=no name=bridge2-iptv
/interface ethernet
set [ find default-name=ether1 ] disable-running-check=no name=ether1-lan
set [ find default-name=ether1 ] name=ether2-lan
set [ find default-name=ether3 ] name=ether3-lan
set [ find default-name=ether4 ] name=ether4-lan
set [ find default-name=ether5 ] name=ether5-lan
set [ find default-name=ether2 ] name=ether6-wan
/interface pppoe-client
add add-default-route=yes disabled=no interface=ether6-wan name=pppoe-out1 use-peer-dns=yes user=012345678@adsl
/interface vlan
add interface=ether6-wan name=vlan2501 vlan-id=2501
/ip dhcp-client option
add code=12 name=option12 value=0x0000041300056800112345678A9101138
add code=60 name=option60 value=0x00001f3901534c46394c7441446a59596977674d30f0056800112345678A9148c5b97e30056800112345678A9136721944009ca15413bd0dd0056800112345678A91
/interface bridge port
add bridge=bridge1-lan interface=ether1-lan
add bridge=bridge1-lan interface=ether2-lan
add bridge=bridge1-lan interface=ether3-lan
add bridge=bridge1-lan interface=ether4-lan
add bridge=bridge1-lan interface=ether5-lan
add bridge=bridge2-iptv interface=vlan2501
****************************************************
Here is the issue
****************************************************
/ip dhcp-client
add add-default-route=no dhcp-options=option12,option60 interface=bridge2-iptv use-peer-dns=no use-peer-ntp=no
****************************************************
/routing igmp-proxy interface
add alternative-subnets=0.0.0.0/0 interface=bridge2-iptv upstream=yes
add interface=bridge1-lan threshold=2

In case you haven't already, best to create ticket to support including supout.rif so they can investigate.

Thanks，I'll do that when I get home.