import bpy

obj = bpy.context.object

rot = obj.rotation_euler
print("store object rotation", rot)
#evaluates correct

bpy.ops.object.transform_apply(location=False, rotation=True, scale=False)
print("rot after rotation-apply", rot)
#problem: rot has changed to (0, 0, 0)

#problem: rotation cant be set back.
obj.rotation_euler = (rot)
print("gameobject_final_rotation: ",  obj.rotation_euler)