How are your preview settings in the config.php file look like? Have you explicitly set the “enabledPreviewProviders” parameter or are you using the default settings, like:
'enabledPreviewProviders' => array(
'OC\Preview\PNG',
'OC\Preview\JPEG',
'OC\Preview\GIF',
'OC\Preview\HEIC',
'OC\Preview\BMP',
'OC\Preview\XBitmap',
'OC\Preview\MP3',
'OC\Preview\TXT',
'OC\Preview\MarkDown'
),
Could you find any related message in your Nextcloud log file if you run a scan command (make sure that “'loglevel' => 0,” is set)?
BTW, you can most likely speed-up your tests if you place some HEIC files in a dedicated directory and use the preview:generate --path=<path-to-dedicated-dir>" command as documented in the command help.