I GROUTED this Shower and it FAILED! -- Why Did it Turn BLACK??

In 2014, I grouted this shower and used Custom Building Products FUSION grout. At the time, I don't recall seeing any specific instructions regarding pebble tiles only that it wasn't to be used in joints over 1/2" wide. For reference, sanded grout calls for using between 1/8" and 1/2" so at the time I figured Fusion would be fine.
I admittedly did not download and read the data sheet on the product before we used it, I only read the instruction on the tub. I also cannot state for certain as to what I read, because it was almost 5 years ago and I could be wrong.
The issue I am having is that the grout is working fine in the dry areas. No staining, cracking, or softening is happening in any of the dry areas. The blackening and softening of the fusion is also happening in the 1/16" joints that are in the wall tile in the wet areas so it has nothing to do with the pebbles.
It took us 2-1/2 days to scrape out the grout (being very careful to not disrupt the kerdi membrane) and 1/2 day to grout with matching epoxy grout.
The customer is happy, so I guess that's all that matters in the end. Some days you make money, some days you make friends.

Видео I GROUTED this Shower and it FAILED! -- Why Did it Turn BLACK?? канала TileCoach