It's possible I don't understand what's really happening here, but this is what it looks like.

I have a laptop with a wired and wireless interface.  I have my DHCP server set up to assign fixed IP addresses by MAC, and it is configured to assign the same fixed IP address for both the wired and wireless MAC.  More precisely, dhcpd.conf matches the MACs to the same hostname, which is listed in DNS with the fixed IP.

The intention here is to make the system available under the same IP address whether it is using the wired or wireless interface, i. e. clients which use the system are ignorant of how the laptop is communicating.

The problem occurs when both interfaces are active simultabeously, with the wired interface controlled by ifplugd and the wireless controlled by NM.

The wired interface appears to come up first, and gets the desired IP address.  NM then goes into a timed loop doing DHCPREQUEST for the same IP address.  The DHCP server is happy to oblige, and responds with a DHCPACK, but NM isn't having any of it, waits about a second, and goes through the process again.  Basically, my DHCP server is being spammed at 1-second intervals all day long.

I assume that it is NM detecting that the IP address is in use by the wired interface, and refusing to use it.

This all worked fine under ifplugd because the ifcfg files had a METRIC of 10 for the wired and 35 for the wireless, so if both were active, the wireless was effectively ignored.  I'm not certain what takes the place of METRIC in the NM world.