Looks like it has an old DBO file associated. Just delete "Skyscraper 2.dbo" from the "entitybank\Buildings Pack\Insides\Skyscrapers" folder and test game + export your game, you should get your stairs back
Do let me know if you find more!
PC SPECS: Windows 10 64-bit, Intel Core i7-8700K, NVIDIA Geforce GTX 1080 GPU, 16GB SYSTEM RAM