It seems our experiences or interpretation of events in game differed then. Maybe it was due to my lack of awareness, which might be possible.
Therefore, I decided to test enemy attack patterns while paying attention. I singled out the enemies, so I could test their behaviour a bit.
Wengari Raiders initiate their attacks only while on my screen. Around the border on the top or bottom, about half-way to the left or right of my character. If I move away, they follow before making their next attack. But: The range of their projectiles is quite high. It is totally possible that they throw an axe while you are moving inside their range, but you get hit when the projectile reaches you a screen away. Correction: they can barely attack me from off-screen from the bottom. Was difficult to tell at first because of the action bar.
Wengari Outriders can throw their bigger, slower, and more visible axes from just outside the bottom of my screen, but they are always on-screen from all other angles.
Some other projectile enemies I tested: Drowned Ravens, Sleet Weavers, Imperial Arbalests, Mortal Hunters - they all can attack me from just outside of my visible range at the bottom of my screen, but not from any other angle, it seems.
Scalebane Sorcerers, Lake Beetles, Winged Fires, Skullen Shamans and Chucklers - always the same. They can attack me from just outside the bottom of my screen, the Lake Beetle’s projectile being difficult to see. Meruna Sirens seem to have a tiny bit more range, but not much. Both their ball attack and wave are highly visible, though. Reacting in time can be tricky, still.
Osprian Vanguard’s volcanic orb is on screen.
Ice Elementals - they start their barrage of icicles when outside the screen, as I remembered. Also, their attack has quite some wind-up time, so if you have left one alive behind you, the icicles might catch you later.
Crystal Elementals - same, only from the bottom can they off-screen attack you.
Scalebane Saboteurs throw spears from a bit further away, but they stick a second to the ground before exploding, so I consider it telegraphed.
Finally, Osprian Lightmages - sometimes they cast their “meteor” from further away, quite telegraphed. But their medium ranged attack, the lightwave, doesn’t even have the range to attack you from outside the screen, it disappears rather quickly.
Now, for the ease of testing, I only ran un-empowered monoliths. AFAIK empowered only scales the health and damage, not stuff like attack range, projectile speed and range, etc.
I tested two different resolutions/ratios: 1920x1200 and 2560x1080, which didn’t make a difference for the north-south visibility.
For the moment, I would conclude that being attacked from the south can result in off-screen kills, but the problem isn’t as dire as like they attack from a whole screen away without telegraphs. Maybe EHG should adjust the attack range of enemies by 1m or so because of the camera angle.
This wasn’t all enemies, of course, but what I could manage for now.