Problem definition :

It is currently not possible to specify the startAt parameter of changelog and paginate correctly for the changelog if the results are >100  , we have a hardcoded limit of 100

{}The startAt for the search endpoint is for issue pagination, this won't take effect on the changelog pagination.

Suggested Solution::
Remove the hardcoded limit of expand results on changelog{}

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Workaround: 

Get changelogs via GET /rest/api/3/issue/{issueIdOrKey}/changelog
https://developer.atlassian.com/cloud/jira/platform/rest/v2/api-group-issues/#api-rest-api-2-issue-issueidorkey-changelog-get