I am not sure if this is intentional or not but it seems like fill mode does not work when you choose two pages. I am on the desktop version on MacOS.
I tested other options in the “Configure Readers” setting. If you choose “one page” with “Continues”, it automatically fill the screen. Whats the point of paying for the subscription to access the fill option if you can just get it for free for “one page” and not working at all for “two pages”?
Also in the “Configure Readers” menu, if you don’t make any changes it won’t let you leave the menu, the save button does not work if there’s no changes made. You have to quit the app and reopen. And when you choose “Continues”, the “Number of Pages” and “Page fitting” options turns grey even after you change the “Continues” back to “Paginated”. You have to leave the menu and entire again in order to access the these options. The UX is really janky with the whole “Configure Readers” part.