One text file is not being backed up by Time Machine
There are a couple of things you can do to narrow down if the system is being told to ignore that file or if there is a problem with the disk structures or some other bug at play.

If you like programs with icons and controls - go get Backup Loupe and use it to track that file across your backups. The program will pause itself if Time Machine is running, so you might do one backup and then turn Time Machine off for the time it takes the Loupe to scan a few backups.

Similarly, Apple has a diagnostic tool that they use to capture information that could show a bug or a problem with time machine and spotlight indexing - it's called tmdiagnose. If you are comfortable with zip files with lots of diagnostic data and text files, start here.

Lastly, you could run tmutil compare from the terminal to just let the system tell you which files are in need of a backup the next "time the machine" runs.

As written, you have four places to look to narrow things down:


Time Machine is being told to ignore that file and you can learn all the exclusion rules and patterns and check for that.
Spotlight is messing up and Time Machine depends on that to know what has changed.
Time Machine itself is messing up.
The disks / hardware / file system are messing up - general troubleshooting of that might be needed once you've ruled out the above three potential causes.