This is a very common problem. The WordPress flush_rewrite_rules() function used in many plugins and themes adds the default WordPress Rewrite htaccess code at the bottom of your root htaccess file (auto_.htaccess). This causes your root htaccess file to be quarantined repeatedly.
To prevent this issue from reoccurring > Go to htaccess File Options > htaccess File Editor tab page > click the Lock htaccess file button and the Turn On AutoLock button. Delete the auto_.htaccess (root htaccess) file in Quarantine.
If you are unable to lock your root htaccess file (DSO server type or host does not allow locking the root htaccess file) then use the alternative method to prevent your root htaccess file from being quarantined repeatedly > https://forum.ait-pro.com/forums/topic/wp-rewrite-loop-added-to-bottom-of-htaccess-file-on-every-site-after-wordpress-core-5-6-update/#post-39821
The default WordPress Rewrite htaccess code
# BEGIN WordPress
# The directives (lines) between "BEGIN WordPress" and "END WordPress" are
# dynamically generated, and should only be modified via WordPress filters.
# Any changes to the directives between these markers will be overwritten.
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# END WordPress
.
