If you click the edit icon for WAN 2 you'll see the static IP is saved. On the main status page it won't show up unless it becomes usable/starts working. I would confirm the circuit is working. Have you tested by connecting a host to the WAN 2 provider directly with that static IP?
The interface could be failing to come up due to ARP failures or something else related to dashboard connection monitor. Sometimes the provider equipment learns the MAC of the connected host and needs to be reset to see a new connection. Or, if possible maybe simply reboot the MX to see if that makes a difference.