Question: Time Machine erroneously reporting it has not backed up in 90+ days
I see this pop up from time to time; perhaps twice per month. Just now I had a notification pop up that said Time Machine had not backed up in 98 days. I go over to Time Machine preferences and it says the last successful backup was in November 2017 (it's currently March 2018, for reference). However, I know for a fact that it has been regularly backing up throughout the past four months.
Anyway, I tell it to manually go ahead and backup now, and it takes a few minutes to do so. It'll then report the last successful backup was today at 10:37am. All seems well, but sometime within the next few weeks it'll do the same thing; say it has not backed up since November 2017. I'm honestly puzzled why this is happening. I've attached a screenshot showing it just a few minutes after it told me it had not backed anything up since November 2017.
In November 2017 I switched from using a USB 3 backup disk to backing up over the network to a Synology NAS. The backups appear to work just fine, though I'll note I never had this issue when using the previous backup disk. I can open Time Machine right now and peruse the various states of my disk all the way back to November 2017 without issue.
I am running High Sierra 10.13.3 if that helps. Has anyone else experienced this issue? Or have any tips for what might help fix this? Thank you in advance.
iMac, macOS High Sierra (10.13.3), null