Randomly missing photos

I am having a recurring issue in Lightrooom Classic. I am using the latest version (10.2), but this has occurred in previous versions, too. I am using an iMac desktop, and my library is stored on an external Seagate drive that's always connected.

I will do a search for missing photos. Anywhere between a dozen to 1200 or so will be listed, out of a catalog of about 144,000. The previews are there, but it says original is missing. So I go back in time machine, find the date range of the missing photos, restore, and the originals are back. This has happened numerous times, and it is usually photos from 2015 and 2020, and often not the same photos from those time periods.

I am no doing anything to the photos; for example, I haven't touched the 2015 photos in a long time.

Any ideas as to why this would be happening?