Make a copy of the sample "11 - Dropdown Menus" scene
Set it up for mobile portrait use ie on the Canvas Scaler change the reference resolution from 640x360 to 360x640 and slide Match from Width to Height.
On the bottom dropdown add 7 more items by pressing the + button on the options 7 times.
Change the Max Height on the dropdown from 200 to 500.
Press play. The first time the dropdown is pressed, the bottom list items are concealed below the screen. The second time the dropdown is pressed, all the list items are correctly displayed.
This happens in the Editor and on an Android device.