Steps to reproduce:

1. Open attached project (test-portrait.zip)
2. Switch to iOS
3. Make sure that 'Orientation' is set to 'Auto Rotation' with only 'Portrait' and 'Portrait Upside Down' as allowed orientations (in Player Settings/Resolution and Presentation)
4. Build the project
5. Run on iOS device while holding it in Landscape orientation

Expected result: Storyboard should be displayed in Portrait or Portrait Upside Down as those are the only allowed orientations
Actual result: Storyboard starts in 'Portrait' mode but rotates to landscape after a while, sometimes keeping even the game view in landscape orientation until the device is rotated to Portrait.

Reproduces with built in launch screen storyboards and custom storyboards.

Reproduced with: 2018.4.26f1, 2019.4.5f1, 2020.2.0a20

Workaround for iOS versions 12 and older: comment out the custom application:supportedInterfaceOrientationsForWindow implementation in UnityAppController.mm file.

Tested and reproduced on:
iPhone 8, iOS 11.0.1
iPhone 6, iOS 12.2
iPad Pro, iOS 12.0
iPhone SE (2nd gen), iOS 13.4