Intermitent failures come and go as they wish. We had so many TCAS inop indications on the early models we ignored them because they were a nuisance and constant distraction. If they had a corroded RA altimiter antenna, it might have worked a lot of the time then failed whenever it felt like it. It is hard to troubleshoot because on the ground the temps are warm and impossible to duplicate. I can see the crews prior to this flight not writing up the fault if it happened a lot.