Hi all,
it seems that the root cause for this issue is identified:
Unfortunately it's not a bug, but a bufix which is responsible for this changed behaviour of autocomplete function in Safari browser.
The bugfix was introduced in iOS 6 and corrects that password input elements on HTML forms with the autocomplete attribute set to "off" were being autocompleted although they should not.
You can check if this is also the root cause in your case by searching in the source code of the webpage with the HTML form if the autocomplete tag is present and set to "off":
<input type="Password" name="password" ... autocomplete="off" ... >
References:
[1] http://support.apple.com/kb/HT5503 (search for string "CVE-2012-0680")
[2] http://www.cvedetails.com/cve/CVE-2012-0680/