I had some time to look into it this afternoon. The problem isn't related to wifi but to DHCP. When you set a static lease on the networkmap, the whole network subsystem is restarted so these changes can be immediately applied to the network clients, causing the client to disconnect. When it tries to reconnect, there's a conflict between the old, still existing lease, and the new IP you're trying to use on the same MAC. If you look in your system log, you will see the DHCP error.
This is nothing new, networkmap has behaved like this for a long time, and nothing has been changed to it in this 380.63. Working around this limitation isn't simple, short of having dnsmasq forget ALL existing leases, or no longer immediately apply any change and require you to manually disconnect and reconnect your client. The latter would be the most sensible thing to do, but it will require analyzing that portion of the code to determine the side-effect of doing so (I didn't write that part of the code).