Thanks for the response.  I'll try to address your questions briefly and do more testing.

1. ISP is Comcast/XFinity.  Longtime solid connection here, should be a non-issue.  (My router is using Cloudflare's DNS, also working fine for a long while.)

2. I'll have to draw something better, but it's:

cable modem->(cat5)->home router WAN

home router LAN->(cat5)->main Deco->(wifi)->slave Deco

slave Deco->(wifi)->laptop and other clients

slave Deco->(cat5)->hub->entertainment stuff + Arlo

Is this a supported configuration?  (Basically I've dropped a Deco in place where I had a wifi client bridge (but no AP) before.  To be clear, the old configuration was solid and known-good.  The only change is using the Decos.)

3. When DHCP requests fail on WiFi re-connect, the laptop assigns itself a non-routed address, in this case 169.254.17.195.

4. I will try swapping Decos.  Should I use the app to properly remove them first, and then re-add them?  Or just physically swap them and let them figure it out?

5. Using wireshark on the laptop you can see the failed DHCP requests asking for the existing leased LAN IP.  The requests get no response from the home router (sole DHCP server).   You can see in this screen capture below what a failed DHCP request looks like on the laptop in wireshark:

I've scrolled the expanded part of the DHCP Discover packet so you can see that the laptop is requesting it's former LAN IP of 192.168.1.143.  In this case the lease is not expired.  (You can't tell from the image but the requests with source IP 0.0.0.0 do indeed belong to the MAC address of the laptop.)

Here's a later segment of the same session capture in which the DHCP request is successful.  In this case because the Deco has rebooted.  In this test I disabled laptop WiFi and waited until the reboot was complete, I'm 95% certain the success happened through the same slave Deco:

However in today's testing I see that the problem isn't just with DHCP.   Later (after successfully getting its DHCP address) the laptop seemed to drop off the internet.  Packet capture shows that packets just aren't making it out past the client Deco to reach the home router:

What you see here is traffic from three Decos (.215, .250, and .230).  (I added a third Deco to the mix.)  The laptop is using it's DHCP-assigned address, but gets no response to its ARP request for the home router.  At that point I can only ping the connected Deco's IP, but not the other Decos or the home router.  And yet you can see there is incoming broadcast traffic from the other two Decos.

For the record I did try swapping the Decos, with identical results.  (So I really don't think it's an individual Deco hardware problem.)

Also it's clear at this point that my thread is misnamed - the loss of DHCP is just a symptom of losing all outbound communication between the satellite deco's wifi client and anything beyond the satellite.  IOW, I can ping other clients of the same satellite, but can't ping the main deco or the router.  Yet I still see (at the satellite deco's wifi client) broadcast traffic arriving from the main deco and the router.  It's as if the wireless backhaul uplink from satellite to main deco is dead, but the downlink is Ok.

It's a bit frustrating that during this time both deco's show happy green LEDs and the Deco app shows each deco is healthy.  There's no other diagostic information available from the decos.

Also this thread is not visible when I visit the deco community forum. Is it hidden or quarantined for some reason?

@MeshedPotato Did you end up getting this resolved with newer firmware?

@LG86 I spent a solid week at it and in the end returned the set.  I got zero indication of what the problem was (or could be).  Good luck to you!

@MeshedPotato thanks for sharing your post.  It appears there are numerous posts in the community forums regarding issues with DHCP and connectivity.  After the last firmware update, I'm literally having rolling outages for devices throughout the house and the system logs are showing numerous errors with IP addresses being handed out and/or cleared.  Although I've had the Deco M9's for a year or so, I'm going to do the same thing as you and take them back.  Thank goodness for Costco's legendary return policy!  TP-Link really needs to read a customer service manual. There have been bug reports on some of these issues over a year ago. Terrible!