The ending is always so rushed afterwards. It goes from Anna/King friendship then fight, Tuptim presumably being put to death (Ken Watanabe was extremely effective with that whip), and Anna wanting to leave only to have the King get suddenly sick due to heartbreak somehow and dying.
That is a good observation. The ending is rushed. We go right from the high of the polka to the low of the whipping. And then all of a sudden the King is mysteriously dying.
Then again the master, Mr. Hammerstein, might have been right on target. The last minutes of the musical do command your absolute attention, and the ending I think is effective.