Steps to reproduce the issue

Cronjob triggers Smart Search plugin to index the content:
php /home/[...........]/public_html/otherdomain.com/cli/finder_indexer.php

Note that this is an add-on domain (since my provider allows me to host several domains - hence the path includes 'otherdomain.com', and of course the relevant cli directory is under that level).

Expected result

All batches processed without any errors

Actual result

Starting Indexer
Setting up Smart Search plugins
Setup 115 items in 0.34 seconds.
JLIB_APPLICATION_ERROR_INVALID_CONTROLLER

Warning: Use of undefined constant JPATH_COMPONENT - assumed 'JPATH_COMPONENT' (this will throw an Error in a future version of PHP) in /home/...../public_html/otherdomain.com/libraries/src/MVC/Controller/BaseController.php on line 232

System information (as much as possible)

Joomla 3.9.18
PHP 7.2.29

Additional comments

Note that this is an add-on domain (since my provider allows me to host several domains - hence the path includes 'otherdomain.com', and of course the relevant cli directory is under that level).

This happens either as a cron job, or directly by ssh into the server.

I tried this suggestion from #24607:
Why not adding an additional
define('JPATH_COMPONENT', JPATH_COMPONENT_ADMINISTRATOR);
inside cli script.

That gave me the following error:
Starting Indexer
Setting up Smart Search plugins
Setup 115 items in 0.242 seconds.
JLIB_APPLICATION_ERROR_INVALID_CONTROLLER_CLASS
There was no WARNING: output.