Try generating a normal map using Photoshop based on one of the textures in the Examples repo (that has a normal map) and compare the output against one of the known working maps, i.e.:
https://github.com/photonstorm/phaser3-examples/blob/master/public/assets/textures/alien-metal.jpg
and
https://github.com/photonstorm/phaser3-examples/blob/master/public/assets/textures/alien-metal-n.jpg
What does the Photoshop normal map look like? Is it like the alien-metal-n.jpg ?