There are 2 appearances, but they're not 6th word.
I have the same issue with reading the second word ''unjumbled row by row'', but how can it be done without skipping some rows (it's shorter than the length of the grid)? But I have copied the word across in the same arrangement as before, so that a particular letter doesn't appear too soon.
My outlines are 6th word, and they contain the non-6th word gaps (my outlines don't go to the edges). All ensuing gaps are within original (6th word) bars and real words are formed - apart from the shaded area (but that is referred to as a 'light' in the preamble).
I'm no mathematician - I don't like maths or maths puzzles, so take it all with a hefty pinch of salt.