Breaking the WP Theme File Editor loopback test

My client needs to be able to edit theme files for the current theme via the standard WordPress Theme File Editor (Appearance > Theme File Editor). When changing PHP files WordPress is not able to perform the loopback test to confirm that the change does not break the site, and therefore reverts the change. I receive an error “Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.”

If I untick “Enable caching” in WP Super Cache and try again the loopback test is successful and I am able to successfully edit PHP files. Therefore it obviously seems to be something related to WP Super Cache or the .htaccess rules created. I have setup WP Super Cache to “Enable caching for all visitors”.

This is a known issue, or do you have any solutions? I can’t see anyway to bypass caching for localhost requests which may be the solution, are you able to please help me write a RewriteCond for htaccess if so? I tried the following inside each of the 4 rewrite blocks but that didn’t seem to work:

RewriteCond %{REMOTE_ADDR} !^127\.0\.0\.1

Thanks ??