So I fixed this myself with two new conditions in a custom JS file that uses jquery within the WP admin.
// run the code when the specific tab of the form group is clicked on
jQuery(document).on('click','*[data-key="field_123456789"]', function(event){
runCustomCodeFunction();
});
// also run my code if the page loads and this tab is visible (it isn't by default as it isn't the first tab)
if ( jQuery(".acf-field-123456789:visible").length > 0 ) {
runCustomCodeFunction();
}