I do not understand why it is fortis in basic ˈbeɪsɪk. I thought that after diphthongs it was always fortis.
The best I can say in reply is that your supposed rule is wrong. In English, unfortunately, the pronunciation cannot reliably be determined from the spelling. That applies particularly to the matter of s versus z with the spelling s.
Compare phase with z and base with s. Or, for that matter, erase with z in BrE but s in AmE.
Although the spelling z or zz reliably signals z (except in a few borrowed words), while c before i, e, y may correspond to s but never to z, even double ss sometimes corresponds to z rather than s, as in scissors ˈsɪzəz and possessions pəˈzeʃn̩z.
That’s why you should always look up the pronunciation in a suitable dictionary, and learn each new word with its pronunciation.