There have been many cases of soldiers being tracked because of phones they were carrying. There was the widely publicized case of Strava revealing where some soldiers were stationed, and recently in Ukraine people noticed that Google Maps was showing "traffic jams" where Russian convoys were massing.

Militaries should probably prohibit personal smartphones completely. But I imagine such a prohibition would be a pretty big hit to morale and would also be very difficult to enforce, especially for non-combat areas where soldiers might not see the point.