Well, the phrase does appear to have some (small) usage in both Tanzania and Kenya, according to a google search. Usually not standalone, and I had to eliminate a couple of artists from the search because it's apparently the name of a song, but there were instances of its use in the wild.
I wrote 'All the best!, Goodbye!' The 'correct' answer was given as 'All the best!, Goodbye!' - can anyone see the difference? What do I have to put to get past this question? This module takes the cake for rejecting its own synonyms and other egregious inconsistencies.