Google has made a small but significant change to Chrome that alters how individual tabs appear on Android. By default, individual tabs will no longer live alongside apps in Android's app switcher; instead, they'll exist entirely inside the Chrome app, as they did before Google broke them out with Lollipop in 2014. Android Central noticed the change, and we've also seen this behavior on new and reset phones.
You can still change the option back
Chrome still has the option to combine tabs with apps, but it's no longer enabled by default. That means anyone with an existing Android phone won't see any changes; someone setting up a new phone might, though they can change it back to the combined style if they want.
Though unchecking a single box in Chrome's settings seems like it should be a pretty minor change, this decision really alters the feel of visiting a website on Android. The initial change had been meant, in part, to blur the lines between websites and apps. Undoing it suggests that Google has decided instead to separate them; alternatively, it could simply be that managing tabs inside the app switcher wasn't an ideal user experience.
Chrome tabs in the app switcher vs. tabs within Chrome.