Greetings from Syncfusion.
We have validated with the simple combobox sample and we are unable to reproduce the reported issue in real device. We have provided the working sample of combobox below. Please have the same from the below link.
Please try our above sample and check whether you could reproduce the reported issue from your side.
Also Ensure the following in your project
1. Ensure whether you are using the common Xamarin.Forms version (above 184.108.40.2066417) for all the project (PCL and iOS project).
2. Ensure you have included Syncfusion.Xamarin.Core NuGet in iOS Project