Very strange. Is this a standard release version of the FBA Pack, or a local build that you’ve made changes to? If it is the standard release, then i’m really not sure what’s going on here - this is the first time i’ve heard of this issue. If it’s a customized version of the FBA Pack, then i’d suggest you take a look at your changes, as well as install the standard release version to see if it works properly.
The one thing you mentioned is that you can’t see the users in central admin - the FBA Pack isn’t meant to be deployed to Central Admin - only individual site collections - so I wouldn’t expect it to work properly in central admin, but should in the other cases. Also, I was wondering more if the sites where it works/doesn’t work are in different web applications - as the membership provider is configured at the web application level.