I'm applying the statsPeriod query as shown in the docs but no matter what I put for the value, it always returns 90D worth of issues

The statsPeriod parameter for the List a Project's Issues endpoint (/api/0/projects/{organization_id_or_slug}/{project_id_or_slug}/issues/) only controls the stats key of the returned data. It does not affect how many issues get returned and will always return all issues (90D).

The statsPeriod parameter then shows you the occurrences of the issues within the time frame you provide. For example if you pass in 14d, it will then show you all the occurrences of each issue returned within the last 14 days.