I always export my working tab to a CSV I’ve had gitchy things happen with XLS files in terms of number formatting and such. A CSV is exactly as it shows nothing left to excels interpretation
Make sure you save your file before reloading. I have seen DMT select the active tab only randomly but never the last tab. The 100% solution is what Joshua proposed (save as CSV). My personal preference is to save that tab as a new spreadsheet but only when a problem like this persists. You could also try closing DMT and re-launching to see if it finds the first tab of the saved spreadsheet again.