Netffos X9 always starts each app / activity as new
Netffos X9 (and probably all other TP-LINK's phones) comes with a handy battery optimization functionality which actually kills all the applications that are in background 3 minutes after phone's screen turns off.

One need to go to: ⚙ &gt; Battery &gt; Off-screen cleanup and disable this feature for all applications that should stay in background (and eventually be turned off by system default process, i.e. after few hours).

All applications in question were listed and enabled on the above mentioned list. Turning off this feature for all those applications resolved the issue.