This happens all the time with my M9 Plus.

@Dagster  Brand new 3 pack, Firmware 1.1.6 Rel 59376 

Same issue as discribed above - tried forgetting network - no affect.

@Dagster I am seeing the exact same issue with multiple iPhones (models 7 and 11). Connectivity is not affected at all.

I have the same issues. M4 pair with Ethernet backbone. Both decos are up to date. 

- Some devices show "Unavailable for current firmware" for Linked Deco. Disconnect of Wi-Fi or Reboot of decos doesn't fix this. 

- Some devices show Wired connection when they are not wired. It seems to happen overnight like this: Device is connected to main deco then roams to secondary deco when I go to bed (Ethernet backbone), and in the morning becomes set to Wired.  Devices with Wired connection remain *stubbornly* connected to the secondary deco and will no longer roam until decos are rebooted.  Disconnecting and reconnecting Wi-Fi on those devices can sometimes get the Wired to disappear, BUT then the displayed Deco is incorrect - whichever main deco is displayed, it is actually still connected to the secondary. After rebooting it happens again the next day. 
 

edit: Deleting the deco app and reinstalling on iPhone stopped the "Unavailable for current firmware" message for the device I was getting it on, but it appeared on another device

This is a bug that tp-link seems to have with all their hardware, i have had M5 and P9 and both systems will when roaming to a different Deco show at random times 'Unavailable for current firmware'. If i look at this thread it looks like all Deco devices have this issue..

@Kevin_Z Is TP-Link looking into this? Seems to be an issue with multiple (if not every) user(s) and multiple products.

 @Kevin_Z Good morning Kevin, indeed also for the latest firmware 1.4.4 in the decos M5, this error remains and is very frequent.
I ask: Do you know why this happens. Will it have a solution in later Firmware?
Thanks and cheers

@Kevin_ZGood morning Kevin.
Does it bother you a lot to answer my two questions on this topic? Generally speaking, firmware 1.4.4 works satisfactorily for me.
Thanks and regards

Also getting the same error. Just purchased my Deco M5 3 pack yesterday. All were updated to latest firmware. Tried reboot, deleting app etc and not working. Sometimes it shows and sometimes it doesn't. The two devices I've noticed it on at the moment are iPhone 11 Max. Please can you fix this. It's frustrating not being able to see which Deco my devices are connected too. Also it's such a brilliantly designed app - so this is such a let down for it!