Not sure what exactly this bug report is about.
Either that when changing the font setting does not reflect immediately in the timeline view - I was not able to reproduce this.
Or that the fonts of the texts in the two circles in your image don't use the same font - this is currently expected behavior. Why? Because it is not possible to set a global font of an iOS app. I have to manually set the desired font to each and every text element. So I could do that for the date and time on the left. But then what about the tags? What about the "month" header? Yes, I could change that too. But then what about the "Timeline" Text in the top bar. And so on and so on... Where do I stop?
I actually tried to set the right font to as many elements as possible, but for some elements (e.g. the iOS dialogs or the context menus, ...) this is not possible. So there would always remain some text element in the app that does not use the selected font in the app setting.
I checked the Day One app, where you can also select a custom font. In Day One, the custom font is not used at all in the timeline view. Would you prefer this behavior in Diarium as well?