If an enemy flies towards a fleet on coordinates, that enemy flight becomes invisible once the fleet enters warmup. This means that you are unable to see approaching enemy flights on coordinates during warmup.
 
For example: I send a battle fleet to attack a pirate. I send a collector fleet behind. Pirate is defeated and battle fleet immediately enters warmup to return. Enemy sends fleet towards same coordinates, but my battle fleet cannot see it, so I allow my collectors to continue and arrive at the coordinates while my battle fleet warms up. It is only when the collectors arrive that the enemy flight is visible. I return my collectors, they enter warmup and the enemy flight is now invisible again. Very soon the enemy arrives and destroys my collectors, which I would have returned had I seen the enemy's incoming flight.
 
 Whether in warmup or not, any ships that are still located on the coordinates should be able to see incoming enemy flights. With warmup applied, evasive action now takes longer. So the fleet should not be blind while on coordinates.

 Please fix

It is not a bug.