The activity must be an outdoor one, and the temperature needs to be below zero on the map of the activity. The temperature of the internal thermometer does not play any role in it.
O estranho que a atividade foi externa, segue todos os parâmetros!
And what temperature is shown on the map of the activity? Can you share the activity as web link and post the link here?
You have to set the Privacy of the Activity to Everyone, otherwise nobody else than you can see it. And on the screenshot you do not show the map, where the temperature is shown. The temperature values in…
Perhaps you did not have your phone with you, the day where the weather data is missing. Or the mobile data was not turned on. Or the weather data provider was not accessible.
Anyway, it looks like the data that the maps shows, is taken during the start, so I suspect the temperature was not as low when you started at the lower altitude, as when you climbed the hills later. It would be safer stopping the activity when you reach the mountain peak, with the lowest temperature, and then starting a new one (syncing the Edge with the phone, to load the actual wether data - assuming there is any signal in that location).
You can also try editing the activity, changing a minor detail (such as adding a Calorie), and then saving. In my case, the weather data on the map often differs from the weather data I get on my device, making me assume, that it is being loaded from another online source, when the activity is being parsed on the Garmin Connect server. And that happens any time you edit & save the Activity, so there is perhaps a small chance that the data could be loaded retrospectively too. Not a big chance, but worth of trying.