Suddenly, we noticed that in some of our domains the DNS records went missing. We have reconfigured our domain with the correct records again. But, we could not able to find the root cause of this issue. Since no changes have been made from our end.
Greetings,
Thank you for asking.
May I ask if this is related to the process when you were adding your domain name to your Cloudflare account, or rather someime later? 
If it’s a case when you added your domain name and the Cloudflare scan didn’t find all of them, I am not sure why, however there could be a number of reasons.
I could gues, either I don’t know how does it work in the background, however maybe the process in the background while you added your domain name to Cloudflare account and while Cloudflare checking for them timed out due to the Cloudflare sending a good amount of requests to check for any existing DNS records, so it might (not) overwhelm the hosting/DNS server, or the connection was lost, or something else in between?
Nevertheless, if any of them is missing, we can easily add them manually with just a few clicks, or by using the step-by-step instructions from the tutorial article from below:
Otherwise, if you have an option to export them, then you can easily import them to the Cloudflare DNS:
However, there are cases when we might be using some kind of a 3rd-party services like eZoic ads or similar and the newely added or recently modified DNS records are reverted back to the old value or similar with other settings.
Using below instructions, you could inspect and determine how did this event occur, if so:
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