From the install command you posted it looks like you selected php-fpm and not multiphp as you mentioned in the initial post.
I’m doing now a clean install with that command and will get back to you shortly.
[Update after install]
- could not reproduce, phpmyadmin worked properly for the default hostname and for another test subdomain http://hostname/phpmyadmin/ , http://test.hostname/phpmyadmin/
- are you trying to open phpmyadmin directly from the ip addres “http://xx.xx.xx.xx/phpmyadmin/”? if this is the case then it makes sense that you get a 404 error, phpmyadmin is explicitly denied in that instance. (https://github.com/hestiacp/hestiacp/blob/4c813ddb22158622723dbf6f79470e882b2d75c5/install/deb/nginx/unassigned.inc)