I am having the same issue with Huawei P9 (EMUI 4.1) android 6.0
I have tried using the built-in icons and custom .png placed under .icn folder (I had to create .icn under Tasker folder manually, for some reason was not there). Whatever way I used I could make it change the "Notify" icon. The same is with Tasker foreground icon, tried to change that one too, it does not reflect the change whatever icon you choose. Only the default one is shown on all tries.
Has anyone been able to resolve this issue?