Interesting...I've had similar dropout issues with google devices, in particular google homes and hub. I had this with the Orbi mesh router  as well before I switched to the Deco M5. 

So even though you have an M9, maybe it's a similar thing....

I am quite confident that my issue was when the google device was positioned kindof half way inbetween pucks/satellites and there was swapping between 2.4ghz and 5ghz bands...I solved this for the troublesome devices by turning on the guest wifi and then disabling 2.4 ghz on that (i need the 2.4 ghz on the main ssid). Then I setup the devices to be on the 5ghz only guest network and the drop outs went away.

At least that maybe something you can try.

Strange, I've 3 Google Home devices and 3 chromecast, and the connection until now is stable as a rock with 3 Deco M9 Plus units: I, and my family, use them everyday.

Yes, strange I agree. Just to update, as few days ago i swapped the culprit google hub back to the main ssid and turned off guest wifi. No issues at all since then. 

Originally I used the same SSID name on the old orbi setup when i setup the Deco. Perhaps there were some Orbi bugs hanging around still !

Anyway, all good now. 

Perhaps the OP should disconnect the troublesome devices and re connect to the wifi again.

@rookie7799 

I had the same problem but rebooting the entire Deco system seems to have sorted it out.

@mellowism having the same issue here. It's very odd. Will post if I work out what to do about it.