I've revisited this.
Now using drupal 10.0.11, captcha 2.0.5 and image_captcha 2.0.5 on a staging site and the error shows.
Using `drush updb` and `drush cr` does not resolve the issue.
However, now after setting a font I'm no longer seeing the error.
Patch #27 works for our use case.
Our deployment process runs drush deploy, which I understand runs database updates and cache rebuild. However for the sake of completeness I ran drush updb and drush cr, and we are still seeing this issue.
The suggestion from #5 of setting a font did resolve the error.
Clearing the cache resolved this for me.
drush cache:rebuild