I suspect that maybe the tiler who did the work
left a polymer film on the surface when grouting.
This is easy to do on a tile that is not shiny, and
difficult to see on a matte or multitextured surface.
One way you could tell is, assuming you have an
extra tile put away, compare the non-installed tile
to the installed tile and see if the never-used tile
is more glossy.
As a professional who has to get this final bit of
film off job after job, I recognize the film on tiles
in the course of life, while in a commercial bathroom,
in friends homes, etc. I often see this film, and
the dirt that is sticking to it. No matter how much
you wash it, the dirt will stick quickly to the surface.

Generally tiles are so hard and waterproof (the glazed
surface) that is seems unlikely they would be "hard
to clean", unless something else is going on.

As previously mentioned in a response to your post,
bleach isn't the cleaner of choice. Your "dirt" is
the result of grease and oils that spill in the kitchen
onto the floor, then when you mop you combine these
with dirt and mix the solution all around.
Therefore you need a grease-cutting cleaner like
pine-sol or spic-and-span pine or something to break
down this greasy dirt.

I've mentioned sulfamic acid many times in this forum.
It is a relatively safe and easy to use solution. If you
scrub your floor with it, it isn't necessarily a grease
cutter, but it will dissolve any remaining grout film
that may be on your tiles.

BTW, bleach may not hurt a high quality glaze, but
is will damage your grout over time. My basic
instructions to customers about cleaning (keeping it
simple so they will remember) the tiles is "use anything
but bleach".

Your problem is a common one, but is difficult to
diagnose "on-line". You have to do some of the
experimenting as described above.

thetiler