Thanks for the detailed description. I use Smart Lock just like you just have Mi Band 4 and it works flawlessly. However, I know from experience with Smart Lock on Chromebooks that it works for someone and not for others. It's just Google thing, Google problem. In my opinion, any problems have nothing to do with Asus. Even here we see, someone is ok, another is sometimes broken.
To be sure try at least this:
https://zentalk.asus.com/en/discussion/comment/34406/#Comment_34406