It seems like currently only the purgeChangedFiles.php maintenance script purges thumbnails of old versions of the file. action=purge, as well as the API, only seem to purge thumbnails of the current version.

Relevant snippet from WikiFilePage.php:

$this->mFile->purgeCache( [ 'forThumbRefresh' => true ] );

And from purgeChangedFiles.php:

// Purge current version and its thumbnails
$file->purgeCache();
// Purge the old versions and their thumbnails
foreach ( $file->getHistory() as $oldFile ) {
    $oldFile->purgeCache();
}