OK replying to myself, there’s been a change in the way DHCP options are sent.
The previous version sent DHCP options like this:
Magic cookie: DHCP
Option: (53) DHCP Message Type (Discover)
Option: (55) Parameter Request List
Option: (12) Host Name
Option: (61) Client identifier
Option: (255) End
Padding: 000000000000000000000000000000000000000000000000000000
In 7.3.0 it was changed to this:
Magic cookie: DHCP
Option: (12) Host Name
Option: (53) DHCP Message Type (Discover)
Option: (55) Parameter Request List
Option: (61) Client identifier
Option: (255) End
Padding: 000000000000000000000000000000000000000000000000000000
The host name is sent first. It seems Virgin Media require Option 53 to be first. I’ve no idea if this a mikrotik or a VM bug (although there’s basically zero chance of getting them to change it if it is, large ISPs don’t even have a bug report process..) as I couldn’t find anything that said option 53 must be first..
The workaround seems to be to change the DHCP options to clientid instead, which removes the hostname field, so you get:
Magic cookie: DHCP
Option: (53) DHCP Message Type (Discover)
Option: (55) Parameter Request List
Option: (61) Client identifier
Option: (255) End
Padding: 000000000000000000000000000000000000000000000000000000000000000000000000…
This works as before.