It's on our list to be fixed "soon".
To be honest the old behaviour was an unintentional bug as well. If was never intended that you can increase the patrol radius of a plane by merging it with a plane that has an even bigger patrol radius. So the fix will most likely be to display the smallest common patrol radius of the stack and also use that for damage calculation. This also is more realistic, troops operating together with other troops are bound by eachothers limitations and don't magically enhance eachothers limitations. So currently the issue is that the biggest circle is displayed and not the smallest one. Players who don't want to use the smallest one then have to split their stack.
To be honest the old behaviour was an unintentional bug as well. If was never intended that you can increase the patrol radius of a plane by merging it with a plane that has an even bigger patrol radius. So the fix will most likely be to display the smallest common patrol radius of the stack and also use that for damage calculation. This also is more realistic, troops operating together with other troops are bound by eachothers limitations and don't magically enhance eachothers limitations. So currently the issue is that the biggest circle is displayed and not the smallest one. Players who don't want to use the smallest one then have to split their stack.