WordPress URLs not ending with a slash redirect to HTTPs homepage

Question

I have a mystery: for my blog,

• HTTP URLs that do not end with a slash (e.g. http://yal.cc/about) redirect to the HTTPS homepage (https://yal.cc) rather than the HTTPS version the URL.
• HTTP URLs that do end with a slash (e.g. /about/) redirect correctly.
• HTTP URLs that correspond to actual server directories (e.g. /game-tools) also redirect correctly.
• HTTP URLs on a subdomain (e.g. http://ru.yal.cc/about) also redirect correctly.
  (subdomain is a separate WordPress installation in a different directory that uses the same theme/plugins/.htaccess)

My .htaccess is nothing unusual:

# BEGIN rlrssslReallySimpleSSL rsssl_version[4.0.7]
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTPS} !=on [NC]
RewriteCond %{REQUEST_URI} !^/\.well-known/acme-challenge/
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R=301,L]
</IfModule>
# END rlrssslReallySimpleSSL
# BEGIN WordPress
# The directives (lines) between "BEGIN WordPress" and "END WordPress" are
# dynamically generated, and should only be modified via WordPress filters.
# Any changes to the directives between these markers will be overwritten.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Things that I have tried to no success:

• Disabling all plugins
• Removing ReallySimpleSSL's redirect from .htaccess
• Switching the theme to TwentyTwentyOne
• Adding this little hack
• Verifying that "base URL" uses HTTPS (it does)
• Changing permalink format to /%postname% instead of /%postname%/
• Comparing configuration of two websites in Plesk (nothing seems different; "Permanent SEO-safe 301 redirect from HTTP to HTTPS" is not enabled)

Where would I start with debugging this? This proves a mild inconvenience since most of the pre-2019 links to my website were http without trailing slashes, meaning that they are now broken without any way to indicate to the user that their subject of search is still there - a slash away.

Answer 1

So, it's unclear which of the things listed in the question has helped, but I have apparently fixed it.

If I were to guess, the original rule in .htaccess was malformed and had been fixed while I was enabling-disabling it (which would have been prior to making a copy that's in the question) - I did undo the rest of the changes after (incorrectly) verifying whether they worked.