THIS IS A HUAWEI ISSUE! Stock Android works perfectly.
I had such problems with my Huawei P9 as well.
The bluetooth data connection with my car (latest Peugeot 308) was barely possible, too.
I have tried to reinstall, reset, re-initialize… nothing hepled.
The solution was… TO BUY ANOTHER SMARPHONE!
Now I have a bq Aquaris X with pure Android 7.1.1, and everything works perfectly. The Pebble, and the car data link as well. This bq smartphone has also almost double battery life, compared to the Huawei P9. It costs less than 300 EUR here in Germany. I am now a very happy bq customer
2-3 years ago I had very similar issue with the Samsung Galaxy S6 Edge which has similarly crapped Android. And no issues with Nexus 5, Pixel and Moto X Force, which have stock Android.