After upgrade to Nougat phone started forgetting recent apps
I believe this is a feature, not a bug. http://www.androidpolice.com/2016/05/18/android-n-includes-tweak-of-recent-apps-ui-with-clear-all-button-at-top-only-7-apps-included-in-switcher-and-quick-switch-shortcut/

The article says that Google claims that "their user research showed that over 99% of users never accessed an app further back than 7 in the recent apps UI".

Unfortunately, for the thousands of 1%ers, this is an annoying change! I have yet to find a setting to modify it's behaviour, and it's possible that it's simply not configurable.