From parse-server\lib\Controllers\FilesController.js - line 32

const location = await this.adapter.getFileLocation(config, filename);
    await this.adapter.createFile(filename, data, contentType, options);
    return {
      url: location,
      name: filename
    };

Location is a const fed in from getFileLocation, but getFileLocation will always return the location before key generation occurs.

object location created by this.adpter.createFile as an output argument is discarded.
Additionally, getFileLocation will return the presigned item based on the old filename rather than the new file name.

Ex:

generateKey: (filename) => {
  return ${Date.now()}_${filename}; // unique prefix for every filename
}

Uploading "file.jpeg" will result in XXX.s3.amazonaws.com/file.jpeg as a key while uploading to XXX.s3.amazonaws.com/XXXX_file.jpeg

getFileLocation will return the full (optionally presignedUrl) or "https://${this._bucket}.s3.amazonaws.com/${this._bucketPrefix}${fileName}" where fileName is file.jpeg

However createFile will instead return a location of
https://${this._bucket}.s3.${this._region}.amazonaws.com/this._bucketPrefix + this._generateKey(filename)

Until this issue is fixed, generateKey should be considered to be non-functional. Either the filename should be rewritten with the generated Key prior to getFileLocation (and the createFile call should not generate a new key) or the FilesController.js should be wrapped to account for this.