This does not seem to be a problem limited to a specific brand because users of Xiaomi, OnePlus, Samsung, and more are all reporting the issue. It is unclear as to what could be causing the problem or how widespread it is, but it seems to be serious enough where Google has acknowledged it and that they are looking into it.
That being said, it doesn’t seem to be a service issue because users are claiming that they are still able to access Google Assistant by pressing and holding onto the home button, it’s just that the hotword detection seems to have gone a bit wonky. No word on when this issue is expected to be resolved, but if you are experiencing this problem then we guess you’ll just have to wait until a fix can be made.
Interestingly enough this only seems to be affecting smartphones and so far there doesn’t seem to be reports of owners of Google Home who are running into the issue as well.