Black joints become partially white

Hello everyone!

Matching our relatively black 60x60cm tiles, we had our bathrooms grouted in black.

At first, all the grout lines were nice and black.
For some time now, however, some of the grout lines are turning white.
When they get wet in the shower, for example, the grout lines immediately turn black again.
When they dry out, they turn white again.

This whole process is reproducible at will.
What can we do about it now?

As far as I saw back then, ready-mixed grout from a bag was used.

Could possibly be caused by the limey water!?

It's the same with me. It always started with limescale, although typical killers like vinegar and citric acid don't help against it...

We have had a water softening system running since we moved into the house.

We have had a water softener running since we moved into the house.

We still have lime scale though. A system doesn’t flush out everything.

Maybe it's due to aggressive bathroom cleaners?