Given that you’ve clarified the issue doesn’t involve hitting the 1 million line data export limit, there could be multiple other reasons for why certain days are missing from your GA4 daily export.
One possibility could be a delay in data processing. Sometimes it can take up to 72 hours before certain kinds of data appear in your reports. If the missing day is within this window, you might just need to wait a bit longer for the data to appear.
Another potential issue could be related to the set date range. Ensure the dates you’re trying to examine are correctly included in your report’s date range.
Last but not least, you might be dealing with a known issue with GA4 where certain kinds of data don’t always appear accurately. The Google Analytics software updates often, so ensure yours is up to date.
If none of these suggestions solve your problem, you may want to consider contacting Google Analytics Support for further assistance.