I’ve got a similar issue with a loss of Contacts. I has been happening for a while but I first thought the issue was me.

However on a morning I realized the contact of a friend I was texting with the night before was gone. I realized the problem was iCloud.

I reach out to Apple support, and after exhausting all the dummy options, the customer support representative admitted that this was a known issue, but they could not find the root cause. And there was no solution to it.

The asked me to monitor my contacts so that I could find when this deletion happen but I would need to give a few minutes range. Because “an iPhone generates a lot of logs and engineers cannot go through them unless we can pinpoint where it exactly happened”.

I thought that pinpointed the bug within a 8 hours range was already good, but the customer’s representative insisted on being accurate within minutes (saying the engineers said that).

Anyway, as all the hard case that I give to Apple Support, the representative said he would follow-up and call me back but never did. So nothing happened.

I move my contacts to Google and just forgot about it.