Looks like of change of plane, going from a horizontal to a vertical surface, for example. The grout is crackig on an outside corner?

The cracking is most likely caused by differential movement.

As to the structure of the joint, I'm not certain of what's under the tile. If there is wood that is getting wet and swelling, that could be causing the movement. If green (not kiln dried) lumber was used under the curb, that could be causing the movement.

If you used unsanded grout on a too-large joint, grout shrinkage could be causing the cracking.

I'd prefer that your joint have the horizontal tile on top overlapping the vertical tiles on the sides so the grout joint is on the "side" instead of on "top".

Remove the grout and caulk the joint instead. Caulk can flex and handle movement better than rigid grout.

When I see that on the entrance dam/curb, it tells me there is wood framing the dam/curb, with tiles placed over them, without a moisture barrier.

The wood is swelling from moisture, Movement causes grout to crack out.

There should be an unsanded matching caulk. Go to the store and ask them to look it up. Could be a custom order.It will resolve your issue.