I don't buy that it was a normal style leak from some manufacturing defect. That system hasn't changed and they haven't had a failure in it in 20+ landing attempts (other failures yes, just not TEA/TAB ignition issues).
I have to think it is due to the nature of the maximum altitude of the center core causing a temperature or pressure issue.
While the 2nd stage can relight after 6 hours in space the 2nd stage has always been expected to function in space. Stage 1 has always been expected to function in atmosphere. No stage 1 has done what the center core did before yesterday.
Heavy (center core), meco ~88km, separation ~91km speed at separation ~9500 km/h
Iridium 4 (stage 1), meco ~75 km, separation ~78km speed at separation ~7300 km/h
https://i.redd.it/cvo84so8aefy.png has some other first stage altitudes and you'll see most separate between 60 and 70 km.
edit:
https://i.imgur.com/NezZ0Qf.png shows stage 1 continuing above 100km in a couple of cases. So that goes against my theory about altitude.
https://i.redd.it/15p6ll4zjrvz.png is another rev of the combined content.