i thought it was weird as well but it's not on his mate SE.  it was on my wife honor 7x but not on this one.

no i havent, because youtube was already installed on mine by default

Does this occur with every app? What if you sideload an APK from apkmirror?

https://www.amazon.com/ANSEN-Rechargeable-Extended-Portable-Charging/dp/B07CTF4PCY/ref=sr_1_3?s=wireless&ie=UTF8&qid=1530102687&sr=1-3&keywords=7x+battery+case 
pls see the detail introduce

Do you want to test a mate se battery case, you can get a free sample 

https://www.amazon.com/ANSEN-Rechargeable-Extended-Portable-Charging/dp/B07CTF4PCY/ref=sr_1_3?s=wireless&amp;ie=UTF8&amp;qid=1530102687&amp;sr=1-3&amp;keywords=7x+battery+case 
pls see the detail introduce

starts the download but never installs.  has anyone had this issue?

huawei mate SE can't install Youtube app from Play store...