What version of Rasa X are you running? The appearance of the scroll bar in your gif when you click on the dropdown tells me you may be using an older version of Rasa X where we had a UI issue whereby the dropdown was actually appearing below the fold of the page, so if you scroll down you should see it.
Update: This issue should be fixed in the next release of Rasa X!
The dropdown overflowing to the bottom of the page is in fact a bug with the auto-placement of the dropdown component. I thought this fix was already shipped, it should be fixed in the upcoming release of Rasa X!