I literally just today had to kiss 2 months of some folders goodbye because TimeMachine utterly randomly decided to stop backing them up. First it decided to stop backing up the home folder; happened to catch that before reformatting. When later I reformatted.... some folders in ~/Documents were empty. Ironically important ones (“archive - important” :( ). I sleuthed through logs, exception rules, everything: no rhyme or reason, no explanation. It just didn’t back them up.

Pure luck that the last reinstall was 2 months, and that version did back it all up. Lesson learned: don’t trust Time Machine. It’s a nice to have.