Objective c 显示UIModalPresentationFormSheet，但显示为UIModalPresentationPageSheet

我已经在谷歌上搜索了两个多小时，但都没有结果

我试图做的是在屏幕上以表单的形式呈现演练。modalPresentationStyle值在整个层次结构中设置正确，但演练显示为页面工作表

SYNavigationController只是将-shouldAutoRotate和-supportedInterfaceOrientation转发到topViewController

有人知道为什么会发生这种情况吗？我已经包括了下面的代码

NSString *storyboardName = @"MainStoryboard_iPhone";
if(UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) storyboardName = @"MainStoryboard_iPad";

UIStoryboard *storyBoard = [UIStoryboard storyboardWithName:storyboardName bundle:nil];

SYNavigationController *walkThrough = [storyBoard instantiateViewControllerWithIdentifier:@"walkthroughNav"];
walkThrough.modalPresentationStyle = UIModalPresentationFormSheet;

WalkThroughViewController *wtvc = (WalkThroughViewController *)walkThrough.topViewController;
wtvc.showSignIn = YES;

[self presentViewController:walkThrough animated:NO completion:nil];