This is a known issue with the Silverlight Toolkit AutoCompleteBox, and that it designed that way in order to match the native experience on the phone, where no autocomplete suggestions come up in landscape mode because of space.
See the Ref here
문제
The autocomplete box displayed when typing the search text in portrait mode, but is not displayed in landscape mode.
해결책
This is a known issue with the Silverlight Toolkit AutoCompleteBox, and that it designed that way in order to match the native experience on the phone, where no autocomplete suggestions come up in landscape mode because of space.
See the Ref here