Redirecting after updating permalinks for one domain is redirecting ALL domains

So I tried unsuccessfully to find the answer in this forum, and googled extensively but nothing has worked so far. Any help is greatly appreciated.

We have Plesk setup on a dedicated server with approximately 100 websites that are mapped using WordPress MU Domain Mapping under a primary domain, lets say it’s called http://www.primarydomain.com. For the sake of example, we’ll talk about 2 of those mapped websites. These websites are completely independent of each other and should be treated so.

Website A has a the example domain http://www.websiteA.com and Website B has the example domain http://www.websiteB.com.

Website A has a page with the permalink http://www.websiteA.com/bio_someguy.html

Website B has a page with the permalink
http://www.websiteB.com/bio_someguy.html

I’d like to change the permalink http://www.websiteA.com/bio_someguy.html to
http://www.websiteA.com/bio-someguy
and set up a 301 redirect for this change. I have set up the 301 redirect in the htaccess file for primarydomain.com (since there is only one htaccess file)and it successfully redirected, however I later noticed that it was also redirecting for Website B:
http://www.websiteB.com/bio_someguy.html redirecting to
http://www.websiteB.com/bio-someguy
which of course generates a 404 error since there is no page by this name.

The code in the htaccess file looks like this:

# 301 REDIREDIRECTS FOR www.websiteA.com
RewriteCond %{HTTP_HOST} ^websiteA.com$ [nc]
RedirectMatch 301 /bio_someguy.html /bio-someguy

Although I thought I was being specific in the RewriteCond, it seems to be ignoring it and just redirecting any matching content on any domain. Because there are so many domains and so many different pages, I have to be sure that I’m only redirecting on the specified domain. Can anyone tell me how to properly do this? Thanks so much in advance for any help you can provide!

https://wordpress.org/plugins/wordpress-mu-domain-mapping/