Let me try answering the questions.
I have no idea why ‘Z-fighting’ occurs
Nope mining before build floor means that you cannot build the floor.
I did not extend the floor plan over a hole but I did extend the floor plan over the ground that has been mined underneath.
For me this happens only when building floor.
Steps to reproduce
Mine Level 1
Mine Level 0 : directly beneath level 1 keeping floor intact
Click build stone floor
Go to level one where you mined.
Build stone floor on that level.
Tearing does not happen every time, though sometimes when units are building units get lazy and don’t mine the ground before laying stone floor.
Then later units may or may not comeback to mine where the stone floor, and ground mix.