You know how in Android the back button is sometimes not a back button? It sometimes transforms to a close button (modal, image overlay, keyboard, etc.)
With an app built for iOS, these “close on back” was not working
As a web app, such behaviour will also need to be programmed to work on a browser first. Packaging it for Play Store should not grant that behaviour automatically.
(2) Image overlay
For Voyager, back button for image overlay works in browser.
When opening an image viewer, the web app triggers a browser history change like this: