This is at all not related to what was experienced in this other thread: Custom Properties disappearing from part file even after Saving and Checking into PDM Standard!! He describes it as disappeared, though the properties shown were still listed and only their values were removed. That is not my case.
Yesterday I opened an enormous BOM of 437 unique components. Nine of them were sorted top the top for having empty values from custom properties. Each of these nine files had all Custom Properties removed. These were NOT merely emptied of values. They were gone, as if deleted. There were NO ROWS. A handful of specific subconfigurations also lost their Configuration Specific Properties, also gone. Not all subconfigs has lost their properties.
I contacted my VAR. I shared the failed files. There were no 'footprints' or traces of history from which to gather what had happened or how to prevent it. It was escalated to SWx with little expectation of resolution, because it can be easily enough explained as a user actively deleting the rows. We only have 2 users, and neither of us would ever do that.
I had backups. I restored files from an otherwise air-gapped HDD. Backups worked fine. One custom material created since backup needed to be reapplied, but that was all that was missing.
Environment Variables: SWx 2018 SP5, Win 7 Pro, Networked file locations. no PDM in use.
That is, I have no explanation, but I'm back to working normally all the same thanks to timely backups. I am not asking anything. I am sharing a very odd error.