@RolyPolyMacaron 

Deco should work through powerline and this powerline adapter model is what I usually recommend for people who want powerline link.

TL-PA7017P has status lights. When upstairs Satellite Deco shows as offline, what do status lights show on its powerline adapter?

Can you connect something by Ethernet cable to upstairs Satellite Deco to verify when it is offline in Deco app will Ethernet link still work (i.e., what if this is just a false alarm in Deco app)?

If you disconnect upstairs Satellite Deco from its powerline adapter, will Satellite Deco establish WiFi link to Deco mesh and if it does - what level of WiFi signal for backhaul it will report in Deco app? 

Install tpPLC utility and see if it reports status of powerline adapters, use it to verify their health during the outage. https://www.tp-link.com/en/support/download/tl-pa7017p-kit/

If you can disable TL-PA7017P power saving mode through tpPLC utility - do that.

tpPLC utility will report speed of powerline Ethernet link. Compare speed of link to upstairs Deco powerline adapter with the speed of link to another powerline adapter. If link to upstairs powerline adapter has much lower speed, that means there might be issues with electric wiring or what else is plugged to outlets on same circuit breaker.

Finally, does upstairs powerline adapter is on same circuit breaker as one connected to Main Deco?

  @Alexandre. Thanks for the help.

I will take note the next time there is an issue.
I have installed the TP utility on a laptop and plugged in the spare TL-PA7017P and connected the laptop. The utility shows the home plug network. Plugging in the spare plug sometimes changes the network, at times the "main" plug is the one connected to the router, other times the "home" plug is the spare one the laptop is connected to for the utility.

The upstairs plug shows around 400Mbps with the others on 660Mbps.

Disconnecting the upstairs Deco from the homeplug switches the backhaul to wifi with medium signal strengh.

I have disabled power saving mode on all the home plugs now.
I believe the outlets upstairs are on a seperate breaker.

As I typed this there was a brief issue. The TP Deco app showed the upstairs unit as offline. I went to look at it and it was flashing red, then solid red. The TL-PA7017P had all 3 lights on and the TP utility showed no issue. Before I could investigate further the Deco unit reconnected

  @RolyPolyMacaron 

Tell me when next event happens, that time I am not sure if it happened before or after you completed changes. Next time it happens also check if TP utility shows upstairs powerline adapter offline. 

It is strongly recommended to have powerline adapters on the same circuit breaker. Your upstairs powerline adapter may be losing connection briefly and during that time its X55 briefly goes offline while it switches to WiFi backhaul.

You said that while on WiFi backhaul, upstairs X55 shows medium signal strength. Have you thought of not using powerline adapter for upstairs X55 and let it run on WiFi backhaul all the time? 

Satellite Deco prefers to connect to Main Deco, by default. If upstairs X55 is closer to downstairs Satellite Deco than to downstairs Main Deco, you can use connection preference feature in Deco app to configure upstairs Satellite Deco connect to downstairs Satellite Deco, thus improving its WiFi backhaul link speed.

See Part 1 of the following document: How to select a preferred signal source on the Deco App