I have a working hypothesis that there's a bug in the Alexa server code that causes it to...once in a great while...direct output to the wrong IP address, and therefor the wrong Echo/Dot/Tap. I have nothing to base this on other than the occasional reporting of this symptom coupled with 30+ years of software development experience, but it would certainly explain this issue. Of course, if the Alexa devices are designed such that they're incapable of receiving and/or rendering unsolicited output (ie, they simply ignore any audio data from the server unless they're expecting it due to some on-going user-initiated action) then that would kill my hypothesis.