I haven’t seen this commented on, but I may have missed it. I finally updated today, and the end result was that my time machine backup lost all the data after sometime in early September 2017. After trying to find any other possible reason for this, I saw that my husband’s back up, on the same hard drive was fine and current. He had not yet updated the OS. So I used my Carbon Copy Cloner clone and undid the update. My time machine content is back, Just FYI. Wonder if anyone else had this happen?
If this happened in early September, it’s probably not related to anything associated with an OS upgrade, since macOS 10.13 High Sierra didn’t come out until late September (and 10.13.4 was just two weeks ago).
It’s worth checking your backups periodically to make sure both that they’re backing up and that you can restore. I encourage people to do this at least every Friday the 13th as part of International Verify Your Backups Day, which last happened in October 2017. 
https://tidbits.com/2017/10/13/international-verify-your-backups-day/
No, it happened today. This morning, before I updated, the backup had finished at 7:08 AM. After the update, it said the latest update was September! Sorry, I wasn’t more clear. After I used the clone to get back to 10.13.3, all my backups were back!
OK, that’s not good. I freaked briefly when I saw that my last Time Machine was five days ago, but that was because the backup drive was dismounted for some reason. It’s working again now that I brought the drive back online.
If you’re reading this thread, take a look at your last Time Machine backup and let us know if it’s not when it should be.
That’s very weird. How long after this did you rollback? Is it possible it was doing something like … indexing the Time Machine drive? Does it index the time machine drive? Is your TM drive encrypted?
I recommend routine use of Howard Oakley’s The Time Machine Mechanic to check recent Time Machine activity. Download from https://eclecticlight.co/downloads/.
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Wow. thanks for that link, I went only out of curiosity, but ended up downloading nearly everything on that page to add to my utilities. Off to play with my new toys!