I suspect the issue is with the logic you’re using in the template itself. Let’s say the field is called “enable_sidebar” and the options are “enable” & “disable”. Existing posts will not have any value stored for the field “enable_sidebar” until you re-save them, so you need to count the scenario of having no value as equivalent to “enable”.
Something like this should do it:
if (!get_field("enable_sidebar") || get_field("enable_sidebar") == "enable") {
//sidebar here
}
