Did you download the Huawei Health App from the Playstore?
If so, you will be far better off getting the APK version of the app.
You will get the full functionality of the Watch GT.
This is the link I used with the Watch GT3
https://appgallery.cloud.huawei.com/appdl/C10414141You will have to allow for downloading from Unknown Sources
Quite normal
I found this worked the best for me
Could be if its the original Watch GT, the software may not be upto date anymore.
As to why it disconnects only in the evening, possibly a change in settings maybe like do not disturb turning on.
Maybe if you leave the phone in the house somewhere and you moving around, loses Bluetooth connection with the fabric of the building
Difficult to know