Extra Info:
Alexa is ignoring other devices if they belong to the same Amazon Account
Interesting fact, is that Alexa is not listening to other devices, if those devices belong to the same Amazon account. Thats the reason, why each device has to have its own Account.
Further info about some Alexa listening issues / features:
https://venturebeat.com/2019/01/31/why-alexa-usually-wont-respond-when-someone-says-alexa-on-tv/