Hyundai Tucson engine doesn't cut out when car stationary

As a fuel saver the engine should cut out when stationary in traffic, unfourtuently it stopped doing so in the first 6 months from new. On it's first service, last July, I was informed that the reason was due to the pandemic causing low usage of the car preventing the battery from fully charging as a fully charged battery was a requirement for the function to work. (The car had only done 1800 miles.) However, the fault is still there even after a single trip of 300 miles had been undertaken. Whilst the car has still only done 3900 miles, I would have thought that a single 300 mile round trip would been sufficient to fully charge the battery.