Update:  Just a couple hours after writing the above and resetting/re-creating my powerline network from scratch, it's happening again right now.  I have 2, 4-unit networks.  

Since I have 4 9020s and 4 9020Ps I wonder if they would align by model.  That doesn't appear to the the case as in one network the split is 1/3 and the other is 3/1.  So it must have another cause.  Crossing fingers that someone has some ideas/guidance.

@Oblio 

I think what is happening is devices are losing connection causing them to establish connection to what they are now able to see. In the powerline network, there is a designed "main" device in the powerline network.

I wonder if any of the powerline lights turn red or go from green to red (slow connection), then turn off (no connection) only to then reconnect to what the device can see.

Both models you have use the HomePlug AV2 protocol so that should be fine. 

I'm thinking you can try designating a different powerline to be the main hub by powering it all down, then plugging in the "central" adapter first. Another thing to try is to place the adapter on a different outlet where ever they are if there is another outlet available.  Lastly, try and set a different powerline network name different from the default: link

@Tony Thank you.  A clarification and a question please:

1. The first device I'm setting on the network is actually on the ground floor of a 3-story house (I did this one first when I did the factory reset on all 8 the day I originally posted).  Based on what you wrote, would I be better served to have the first/"main" device be on the middle floor (so that the "main" device is in the middle distance-wise between the 2 other devices downstairs and the 5 upstairs)?  

2. If I unplug the first device then plug it back in, does it lose it's place as "main"?  (i.e. does the "main" device role shift to one of the remaining devices) and is this perhaps my problem?  Should the "main" device be one I never unplug?  If the role as main shifts, is there any predictable way that this is happening (i.e. is it accidentally shifting to the one in the garage which would be the worst choice since it's furthest away from anything)?

Thank you.

Sam

@Oblio 

1. Correct, that is what I would try.

2. The main should change to another, which too is not clear. So it may be that having any other be the main, just not the main (near the router) as that is currently what is giving the issue or speculated to be so far.

If in the end, you still get the situation of the powerline devices creating their own segments, I would change the powerline network name to something else. Finally, reach out to support to see what they may find for you.

@Tony Thanks Tony, this is helpful.  

As someone in the software industry I wonder if there is a future product improvement which could be considered where the "main" or "master" unit can be customer-defined to reduce the chance of having the network segmentation problem happen. Perhaps such a feature could be hidden in the 'Advanced' section of configuration.  I also recognize that I'm probably in the tail of deployments (I suspect most homes have just a couple of these things and not 8 of them) so perhaps my scenario isn't a common one.

In the meantime, now that I understand how it works a lot better, I feel I'm better situated to work with what we have today.  Thank you.