I migrated my site from static to wordpress and in interest of maintaining SEO retained the .htm extension on all pages however I have WP installed in folder /blog with following in root htaccess
Options +FollowSymlinks -MultiViews
RewriteEngine on
RewriteCond %{REQUEST_URI} !^/+blog [NC]
RewriteRule ^(.*)$ blog/$1 [L,NE]
Almost everything works perfectly and google is happy as all links are still unchanged except for this single URL https://www.example.com.au/blog/ which instead of displaying https://www.example.com/blog/index.php returns a 404 not found.
The /blog directory has standard wordpress htaccess
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
RewriteCond %{REQUEST_URI} !/index.php [NC]
</IfModule>
I have blundered around for hours with redirects and have tried using the following the /blog .htaccess but all with no success.
RewriteRule ^(.+?)(/[^/]*|)$ $1/index.php$2 [L]
and
RewriteRule . /index.php [L]