I solved the riddle!
I often go through the following procedure at the end of a contest to prepare for the next time I will use that contest log for a new contest. I save the log by a unique file name. Then without exiting the contest log program I "Start a New Log with a New Database File or New File location". Then I close the contest log program. After the 13Z CWT this morning, that's what I did and found that the ContestHistory.txt file did not contain new or updated info even though the file date-time-stamp showed that the file had been modified.
After the 19Z CWT this afternoon, I closed the contest log program without going through that procedure. New and updated info got saved to the ContestHistory.txt file.
I guess I just assumed that the ContestHistory.txt file would be updated when saving the contest file. But that is not the case. The ContestHistory.txt file is updated only when the program is closed. So following the procedure I described above resulted in a new blank contest file being used to update the ContestHistory.txt file.
Scott, I'm kinda curious about why you chose to update the history file upon exiting the program rather than when saving the current contest.