I was working on building a new HTPC today and found that creating a bridge the old way no longer worked.
I initially thought that it was an issue with Network-Manager, but found that Ubuntu have switched to using netplan (I'd never heard of it before!)
So, the quick and dirty way to get it working is as follows!
Use a text editor to edit the file name: /etc/netplan/01-network-manager-all.yaml (it may be just /etc/netplan.yaml on Ubuntu server) Then add all the following and change it depending on your subnet, whether you use DHCPv6 and so on.
Check the attachment 01-network-manager-all.txt for the file containing the changes