This question is an exact duplicate of:
When choosing which sites to pin to show in the left drawer, sometimes the list of suggestions shows inaccurate suggestions or doesn't show up.
To reproduce, open the left drawer, then click the "YOUR SITES EDIT" row (below "Feed"). (1) Tap the "Select a site to pin" spinner's downward facing triangle. (b) The list of suggestions appears and the triangle now points upwards. (2) Click the triangle again. (b) The suggestions list disappears.
Seen behavior: The triangle stays pointing upwards. (3a) Clicking the triangle again has no effect.
Expected behavior: When the list disappears (b) the triangle should then point downwards. (3b) Clicking the triangle again should show the list again.
Note: to recover from this modifying the text on the spinner's textbox shows the dropdown list again.