I think this is an issue with how AngularJS handles string values in the model provided to the <input type="number" ...> field. See: https://stackoverflow.com/questions/16829616/ng-model-for-input-type-number-not-working-angularjs
If you do need to write some Javascript to change the type, I’m not sure whether you have access to $scope, but you should be able to write Javascript in the template after this change: https://github.com/gocd/gocd/pull/5361