This happened to Cloud in https://github.com/elastic/cloud/issues/29553 while setting an Elasticsearch keystore setting for an AWS S3 account's session_token. That value tends to be a very long value and Cloud shows it in an <EuiCode> block within the modal, which causes this behavior:

As noted in the workaround in the Cloud issue, if you remove the text from the modal body, then the buttons come back into the footer's view as expected.