There are two things going “wrong” here, and two ways to fix it.
The first is that apache “figures out” that there is a directory by the name of “portfolio” before the rewrite conditions are applied. That means that the rewrite conditions are receiving “portfolio/” instead of “portfolio”.
Second, the “!-d” rule is specifically avoiding the rewrite that you want to make if there is in fact a directory by that name
Solution 1: Manually re-route requests for the portfolio directory to remove the slash.
# Manually re-route portfolio/ requests to portfolio
RewriteCond %{REQUEST_FILENAME} portfolio/$
RewriteRule ^(.*)/$ $1
# Hide extension
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.*)$ $1.html
Note the removal of the “!-d” condition.
The downside to this is that you are having to hard-code the “portfolio” edge case directly into the rewrite rules, and will still result in the browser being first redirected to portfolio/
Solution 2: Set DirectorySlash Off and remove directory exists test
# Disable Automatic Directory detection
DirectorySlash Off
# Hide extension
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.*)$ $1.html
Setting DirectorySlash Off would fix this issue the best, but may break other parts of your site where you actually want the auto DirectorySlash. Best of Luck, and I hope this helps.
Note when testing solution 2, your browser may remember the redirect of “portfolio” to “portfolio/” and perform the redirect before it even sends the request to the server. Be sure to test in a cache-clear, clean environment for best results.