This is a known issue. It most frequently happens in the event Spell Tiny Words, but could happen in any event.
The problem occurs because the bearaphrase generator doesn't know all of the words in the game dictionary. If the bearaphrase generator doesn't find enough known words to match at least one of the bearaphrase templates, then the bearaphrase is blank.
Players encounter the problem most often in Spell Tiny Words because that event's rules and board layout reward players for making short words. Thus there is a tendency to spell lots of short words--words that are likely not known to the bearaphrase generator. Here is an example of one player's input on Spell Tiny Words that results in a blank bearaphrase:
hi si pa te wote re the na eco si in ko te es re ne