Here's the behavior I isolated with Preflight-enabled apps built today (14 Oct):
- Upload a new brand image, click Save, and go to Contents & Layouts
- Click the Preflight button and allow the progress bar to finish.
On iOS: Force quit and relaunch the app to see the new brand image.
On Android: change views to trigger the Update message, update and you'll see the new brand image. N.B. I found that I had to click on an article or collection to go "one level further into the app" to get the update message rather than switch to another top-level item.
This was with preflight-enabled apps.
I did not need to delete and re-install any apps.
HTH;
Colin