Google is making important changes to Gmail starting February 20
So, starting February 20, multiple inboxes with preview pane configurations will no longer be supported. Those who do use inboxes with such configurations will see a notification in the app, which will let them know of the upcoming change.
When the time comes, the preview pane will turn off whenever you're using multiple boxes. The preview pane feature will continue to be available as long as you switch to an inbox type other than multiple inboxes by heading to Settings / Inbox / Inbox type.
Last but not least, Google plans to introduce a single action toolbar across multiple inboxes, which should help users take actions across multiple sections much easier. The changes will start rolling out on February 20 and should be available to everyone by March 5.