Recovery is rewritten I am using a Lenovo p70 which I flashed with AOSP 6.0 ported from Lenovo k3. 

The ROM keeps overwriting the TWRP custom recovery with stock recovery (which doesn't even boot).
As far as I know this problem is caused by the fact that /system is mounted as read-only. In most cases twrp must promp you and take care about that but that is not my case.
Also I tried editing "fstab.mt6752" but it gets overwritten on boot/reboot and the default values are returned.

So the question is how do I stop the ROM (AOSP) from overwriting the TWRP?
And in case I am right, how do I make system to be mounted as read-write by default?
Don't know if you already done. But, most customs like CM, have an option in developer settings to not overwrite recovery.