I have seen some imported glazed wall tile that has some degree of porosity in its glaze and when subject to moisture can wet out the body of the tile to some degree. Your descriptions of only seeing the streaks when the light hits the wall at certain angles reminds me of a condition called intent fracturing due to excessive shrinkage of the mortar setting bed beneath it.

This is caused by cracking in the setting bed that fractures the bottom of the tile but not the surface of the tile.



Once all of the sealant is removed, an optional step is to clean the stone surface with a quality stone cleaning product. Practice in a hidden area before tackling the main areas, or have it professionally sealed. I would apply more sealer of the same type and evenly apply it over the tile and wipe it dry with the lint free cloth to see if it takes away the streaks.

You can’t feel them and you can only see them from an angle and they can appear to be a dark streak. He was at my shop and took the wrong sealer, he put two coats and it looked good from a distance but there was streaks.

Source: texasstonesealers.com/indoor-floor/

genaMarbleStone
I have seen some imported glazed wall tile that has some degree of porosity in its glaze and when subject to moisture can wet out the body of the tile to some degree. Your descriptions of only seeing the streaks when the light hits the wall at...