Unknown provenance, no idea of actual construction details or if it was actually constructed per the manufacturer's instructions, no idea of what type of grout was used, or cleaning materials, or other maintenance or use, and read this, if you care for some unbiased industry experience on efflorescence and thinsets
https://www.tcnatile.com/faqs/31-efflorescence.html. Latex migration can also leave stains on the surface of the grout, depending on the situation.
John is on a mission...he is the only one that can give information on this site regarding tiling things, or so he thinks in his mind. Things work best when you get various opinions, and sometimes, that means going elsewhere. Different viewpoints, different experiences, different biases. Don't shoot the messenger, take the info, filter it, and try to understand where it is coming from, and why. I have no financial interest in promoting any one thing or myself...John does. So far, he's disagreed with a couple of Ph.D's, the TCNA, and Schluter, saying they essentially don't know what they're talking about. It's no wonder that he finds lots of fodder with me.
It takes a lot of water penetrating the tile and mortar to leach out any salts, and in a properly constructed shower with proper drainage, especially when you're dealing with a membrane, that only leaves the grout and the thinset. There should be VERY little water that ever gets to the setting materials, and very little to effloresce. It takes awhile to totally saturate the gout down to and include the thinset - in a well constructed shower, that moisture rarely gets there, and never in any significant quantities...it evaporates or runs off first. Since most grouts are modified, and there is so little thinset beneath the tile, you really have to do something wrong for it to occur - otherwise, you won't dissolve the very small quantity of salts in the materials in the first place for them to leach to the surface.
FWIW, a modified thinset can use one or more methods to help prevent the salts from leaching, several of which are to add some oil to the thinset or to add some calcium carbonate (don't try this yourself!, leave it to the manufacturer). The modifiers can help some too. If you actually have an install over a mudbed or over a concrete slab, it can be a bigger issue, but John's knocking Schluter products which tell you to use unmodified thinsets over them, so there's only the thinset and grout that can be a source.