I was able to reproduce with another account I have. Looks like new account can't follow proposals.
Here is the link to the account profile page, showing it following the same proposal, but as in your case it's not "real" i.e. not showing anywhere else.
When trying with existing, old time account, all is working fine.
Mystery solved! When you register new account on Area 51 you should get such an email:
Thanks for registering with Stack Exchange Area 51
In order to follow proposals, we need to confirm your email address.
All it takes is a single click.
Check you email, maybe it landed in the spam folder.
(note this applies also for users with accounts on other sites, since Area 51 stands on its own)