Or if their child contains width: '100%'.
Dunno if I'm in that niche territory, but this has taken me a very long time to spot the source of the problem. I hope this can be fixed so that the next person doesn't have to.
Comment out either flexDirection: row or width under headerContent.
https://snack.expo.io/S1Pdpkgh7
And here's the expected behavior:
https://codepen.io/BANANATRANADA/pen/jeXoLj