Angled reflection on flat sprite surface

Hi!

I have a relatively simple problem with UE 5.1:
When I add a Sprite with a flat surface and the default lit material from Paper 2D the light reflection do look angled into one direction, even though I have a flat normal map.

From one side the light is reflected by the white sprites:

From the other side the sprites do not reflect the light:

Has anybody an idea of what could cause the problem and how to fix it?
Thanks!