Two things:

  1. Have you tried using resolution instead of level in your style file:
    amenity=bench [0x004a resolution 24]
    highway=bus_stop [0x2f08 resolution 18]

  2. Garmin GPS units often have their own ideas about what resolutions certain things will appear at, no matter what you stipulate in your style file.

Things are further complicated by the fact that you can alter the detail of the map. Each extra degree of detail you select “moves” everything up one zoom level. Declutter option also affects this, as does the setting where you can decide the minimum zoom level at which POIs occur. But of course these latter settings should affect both POI types equally.