here is a normal example scenario.
You have 3 WordPress sites and each of them has their own .htaccess files. each of those WordPress sites will only follow the rules in its own .htaccess file and WordPress will handle permalink rewriting internally on each of those sites. So something unusual is occuring here to create this issue/problem.
Example:
If all 3 example WordPress sites are using the standard WordPress .htaccess code then each site will ONLY follow the rules in its own .htaccess file and WordPress will do the permalink rewriting internally with php code. WordPress handles permalink rewriting internally and NOT in the standard .htaccess file/code that WordPress generates.
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
If you have added your RedirectMatch code outside of the WordPress Rewrite Loop then it might be possible that this is affecting other sites, but I have never seen or heard of the problem you are describing before when using the standard WordPress .htaccess code/file.
For whatever reason this problem is occurring on your main site your .htaccess file should look like this on the bellydanceoz.com site to prevent this problem:
NOTE: What concerns me about doing this is that WordPress already handles permalink rewriting internally and NOT in the standard WordPress .htaccess code/file. So this may have an undesirable effect since this is very non-standard and is normally already handled by WordPress internally and would not need to be done at all.
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^bellydanceoz\.com [NC]
RewriteRule ^/([0-9]{4})/([0-9]{2})/(.*)$ https://bellydanceoz.com/$3
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
