When adding alertOptions to ion-select with cssClass option … css class is not added to the wrapper as mentioned in Ionic documentation
The bug is in the Alert and not in the Selection. The cssClass is passed correctly but in the Alert it gets overridden by the default classes of the Alert using the Alert method setCssClass. This method does not add classes as documented but really sets them thus overriding existing values.
I have opened an issue for this: https://github.com/driftyco/ionic/issues/6835