Why I miss some data on querying of Chrome UX Report API?

On querying of Chrome UX Report API i get sometimes a 404 error, "chrome ux report data not found". Documentation says: If 404 - CrUX API doesn't have any data for given origin.

For all URLs I query, I get some metrics, there is no URL, where all metrics would be missed, and for most URLs I get all data.

But there are cases, where data of certain metric missed. For one URL is FID data missing (data for all other metrics exist), for another URLs - FID, LCP and CLS are missed (data for FCP exist).

Is it a kind of API glitch? What should I do to get data for all queried metrics?

PS: if i query the same URLs now and after 30 minutes, I get different results: for same URLs are different metrics data missed: at first query is FCP missed, at second query - LCP and CLS... Why is it so?

On the image you see how missed data looks:

FCP is the only metric guaranteed to exist. If a user visits a page but it doesn't have an FCP, CrUX throws it away. It's theoretically possible for some users to experience FCP but not LCP, for example if they navigate away in between events. Newer metrics like CLS weren't implemented in Chrome until relatively recently (2019) so users on much older versions of Chrome will not report any CLS values. There are also periodic metric updates and Chrome may require that metrics reflect the latest implementation in order to be aggregated in CrUX.

The results should be stable for roughly 1 full day. If you're seeing changes after only 30 minutes, it's possible that you happened to catch it during the daily update.