That is disaster bug for users. I had received one similar report before, I spent a whole day try to reproduce but no luck.
I am very doubt it is because some exception occurred during the middle of editing, which stopped the "auto-write" flag and cause all following input were not saved into disk.
Can you open application "console", then searching keyword "TextNut". Find out if there are any exception log during that losing error occurring period? Any error.... one of them could cause the problem.
If too much, please paste them into a text file and send to my email. Thanks.
Sorry for this terrible bug...