Hi!
Me too, same "problem". As if sometimes the default Huawei lockscreen would attribute itself the priority.
I had tried other lockscreens (e.g. Next Lock Screen) and I had to uninstall it because default Huawei lockscreen always came and disturbed...)
The more I use the Mate 9, the more I find it difficult to "tweak" it with other things as launchers or lockscreens, as if Huawei made sure one use only Huawei' own softs...