FWIW, I tested the package in bionic-proposed and it passes the test I describe above:

root@b2:~# netplan generate
Error in network definition //etc/netplan/50-cloud-init.yaml line 19 column 16: unknown key port-priority
root@b2:~# sudo vi /etc/apt/sources.list
root@b2:~# sudo apt update && sudo apt policy nplan 
Hit:1 http://security.ubuntu.com/ubuntu bionic-security InRelease
Get:2 http://archive.ubuntu.com/ubuntu bionic InRelease [235 kB]
Hit:3 http://ppa.launchpad.net/raharper/experimental/ubuntu bionic InRelease       
Hit:4 http://archive.ubuntu.com/ubuntu bionic-updates InRelease                    
Get:5 http://archive.ubuntu.com/ubuntu bionic-proposed InRelease [235 kB]
Hit:6 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Get:7 http://archive.ubuntu.com/ubuntu bionic/universe Sources [9019 kB]
Get:8 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages [1010 kB]
Get:9 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages [8372 kB]
Get:10 http://archive.ubuntu.com/ubuntu bionic/universe Translation-en [4921 kB]
Get:11 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 Packages [146 kB]
Get:12 http://archive.ubuntu.com/ubuntu bionic-proposed/main Translation-en [59.8 kB]
Fetched 24.0 MB in 5s (4439 kB/s)                               
Reading package lists... Done
Building dependency tree       
Reading state information... Done
45 packages can be upgraded. Run 'apt list --upgradable' to see them.
nplan:
  Installed: 0.32
  Candidate: 0.33
  Version table:
     0.33 500
        500 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 Packages
 *** 0.32 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        100 /var/lib/dpkg/status
root@b2:~# sudo apt install nplan 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Suggested packages:
  network-manager | wpasupplicant
The following packages will be upgraded:
  nplan
1 upgraded, 0 newly installed, 0 to remove and 44 not upgraded.
Need to get 41.6 kB of archives.
After this operation, 3072 B of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu bionic-proposed/main amd64 nplan amd64 0.33 [41.6 kB]
Fetched 41.6 kB in 1s (81.1 kB/s)
(Reading database ... 27026 files and directories currently installed.)
Preparing to unpack .../archives/nplan_0.33_amd64.deb ...
Unpacking nplan (0.33) over (0.32) ...
Setting up nplan (0.33) ...
Processing triggers for man-db (2.8.1-1) ...
root@b2:~# cat /etc/netplan/50-cloud-init.yaml 
# This file is generated from information provided by
# the datasource.  Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    version: 2
    ethernets:
        eth0:
            match:
                name: 'eth0'

    bridges:
        br0:
            addresses:
            - 192.168.14.2/24
            interfaces:
            - eth0
            parameters:
                path-cost:
                    eth0: 50
                priority: 22
                port-priority:
                    eth0: 14


root@b2:~# netplan generate
root@b2:~# set -o vi
root@b2:~# cat /run/systemd/network/10-netplan-eth0.network 
[Match]
Name=eth0

[Network]
Bridge=br0
LinkLocalAddressing=no
IPv6AcceptRA=no

[Bridge]
Cost=50
Priority=14