Well the reason I would say it’s not efflorescence is because a symptoms is that the grout is lightened in appearance, not darkened.
As the original colour is smoke grey, which I believe to be quite a light shade, you wouldn’t be getting the dark shades If it was suffering from salt deposits.
I always try and 'peg' the grout joints with these new grey grouts and that seems to stop any shading issues. If the joints are narrow like these, then I would just let the grout firm up a lot and then rub the grout joint with a fluffy towel. That works too.