Excellent.

The journal *was* saving what you wrote, it just was in white ink. For faster performance, the journal immediately draws what you write in a coarse black colour. When you save, or change pages, or tap the Refresh icon, it redraws what you've written in higher quality in the actual colour. So what you wrote initially looked black, but when it gets redrawn it "disappears" since the actual colour is white.

You can disable this optimization by unchecking "Quick Draw" in the Settings.