Hitting esc to pause and go afk, upon returning and resuming time has progressed and mobs are attacking.
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well, it should pause the game unless you have played with other people in that world

It is actually a known issue MCPE-21221.

More Detail, Multi-Player off,
Console Versions, Tested PS4 and Switch, pausing to menu pauses game.
Pocket Versions and new Better Together, Tested iPad current release and Windows 10 1.2.0.2
Pausing to menu does not pause the game, either platform.
Of course in multi-player it would be expected for time to progress for everyone together regardless.
Since this is a known issue and I just missed it when I searched this can be closed/merged into the existing case.

Unfortunately this is NOT a bug. We have been asking for this feature for a while along with disable autosave. Helen Angel the community manager announced on her Twitter that they were adding both features. That was about 2 years ago. Don't hold your breath. I recommend just get used to it and keep your fingers crossed for "someday".
This has always been. Pausing the game doesn't actually pause game.