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.