Fix automatic .htaccess redirect updates

One of my clients has a problem on their WooCommerce website where the .htaccess is being updated with a RewriteCond line of code whenever they publish a custom post type. This causes the website to flag up a 500 error as the line is apparently too long and breaks the site. I can fix by removing the line manually, but it persists when the custom post type is published again. The original developers are not helping to solve the problem, so I need someone to investigate and fix so it doesn't happen!