When you apply a segment to a standard report, you’ll also need to set a date range for the segment – this is the range the segment is available to apply to a report. If the range selected on the report does not overlap with the segment date range, the report will default to the last date available for the segment. The segment date range must be selected when first applying the segment to the report, in this modal (note the "modify" option):
It is recommended that when selecting the date range for a segment, you set the start date far back enough that it covers your reporting needs and set no end date so it will be available when using it again in the future.
The segment will then take some time to process. Once it’s processed, you can immediately apply it to the report by clicking it in the segment dropdown: