I’ve updated the plugin to the latest version (1.8.3) and the Site Health doesn’t show the PHP warning.
However, the issue still persists. I tried to edit the Course and add a new lesson. On saving the Course, I got the same JSON error.
I then deactivated the Tutor LMS Elementor Addons plugin and tried saving the Course again and didn’t get any error.
So, I think the error is caused by the Tutor LMS Elementor addon plugin.