Hi KITTIPAT,
Thanks for using syncfusion products.
We have validated your query and we are able to reproduce the reported issue” When I tap and drag on the image with one finger, it still entering the freehand drawing mode.” in Xamarin Forms iOS platform and We have already fixed this issue and the fix will be included in our 2018 Volume 1 Main release which is excepted to be rolled out by end of this week.
We appreciate your patience until then,
Regards,
Ashwin