Why does my website keep redirecting to http and not https?

On my phpBB forum board, it keeps on sometimes redirecting to the 'not secure' version of the site aka http
Any way to make it where it completely redirects you to https 'secure' url and not the other?
My coding partner already did a force https thingy already but it still is doing the same thing
My website:
https://www.hewan.org/
in .htaccess file

RewriteEngine On 
RewriteCond %{HTTP_HOST} ^hewan.org [NC] 
RewriteCond %{HTTPS} off 
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

[alemoppo]

Try with:

RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteCond %{HTTPS} off
RewriteRule .* https://<your domain>%{REQUEST_URI} [L,R=301]

Bye!

Do I add that onto the following code or remove previous and have this instead?

[alemoppo]

Try removing the previous code.
You should also have to enable the RewriteEngine (if is not active), so:

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteCond %{HTTPS} off
RewriteRule .* https://<your domain>%{REQUEST_URI} [L,R=301]

Bye!

Still redirects to the unsecure http website when I go onto phpBB acp panel....

[alemoppo]

On the homepage, there is content that creates mixed content, and it is: http://www.hewan.org/discord-widget/Discord.css
You should then either include it in HTTPS or remove it.

In general, HTTPS (and the redirect) is delivered correctly on your site, so it depends on the application settings: I would therefore recommend that you ask phpbb support.

Bye!