Hi, Dylan,
There's no problem - the last across clue contains a two-letter thematic cell, plus four single-letter cells, so uses five cells in all. So it matches the first across clue, which has a six-letter thematic cell plus four single-letter cells, again using five cells in all.
This was a bit of a slog to begin with, requiring a fair bit of cold solving, but fortunately I found the clues for the top half of the puzzle more manageable, and most of the extra words were in these clues, so the theme emerged relatively quickly. It's still taken the best part of five hours, though!