I can confirm that this still appears to be an issue.
Tunnelblick 3.5.4 (build 4270.4395) on Mac OS X 10.9.5 (13F1112)
The steps involved in my situation appear to be:
1) installed and launched Tunnelblick
2) added a handful of configurations from my VPN service
3) went straight into VPN Details… > Advanced > VPN Credentials > [X] All configurations use Common credentials
4) opened Mac Keychain Access app, applied 'tunnel' filter
5) connected to first Tunnelblick configuration, was prompted for credentials (dialog says it's using Common credentials), saved to Keychain
6) noticed that a server specific credentials entry was created in Keychain Access
7) disconnected and connected to second configuration
8) again prompted for credentials, saved to Keychain, another server specific entry was created
9) (read this post…) disconnected and quit Tunnelblick then launched it again
10) connected to first configuration again, prompted for credentials again, saved to Keychain again
11) this time noticed that the Tunnelblick-Auth-Group-Common entry was created **SUCCESS**