Yes, that’s normal. Black steel outdoor products change color over time and can develop surface rust, and your garden fireplace is no exception. It has a heat-resistant coating, but that coating is exposed to high temperatures, rain and UV light. As a result, the layer wears down gradually, which can lead to small rust spots or a brownish tint.
This is purely cosmetic and does not affect the performance or lifespan of the fireplace. Want to keep it looking its best? Cover it when not in use, or touch up the affected spots with heat-resistant paint.