TBH, I have no idea what has happened. Originally I thought it was because of a typo I'd made on an input tag.
I fixed the typo, everything worked when tested so I assumed that had been the problem. Now there are some examples which only show data missing from part of the left column and with saves from before that time. If the tag had been the problem it would have affected the whole column as it's generated by repeating that single input tag. There's no way that typo could only affect part of the column.
How data can go missing from sheet that haven't been saved, how it only seems to affect a few sheets I currently have no idea. Even if a backup exists so we could go back and reload I've no idea how to identify which sheets may be affected.