Question: Do you really have more than 20 license pools?
I tested this on my (fairly large) Splunk deployment (with four license pools) and didn't see any gaps over 30 days.
earliest=-30d latest=@d index=_internal source="*license_usage.log" type="RolloverSummary"
| timechart span=1d sum(eval(b/1024/1024/1024/1024)) AS volume_tb BY pool
| rename _time AS Date | eval Date = strftime(Date, "%F")
I would remove the UF and let the LM forward its logs to your indexer(s) and see if your results are more complete after a week or so. Perhaps remove the limit=20 and useother=f options from timechart.