I play on the PS4 and I have never seen the alert message "xxx settlement is under attack." I have had several settlements attacked when I arrived via fast travel and missed at least one because all I saw was a failed message when it was looted. Is this a known bug or is it just me?

I think it's a bug and sometimes it doesn't alert you when a Settlement is under attack.

I think its two different things. The settlement is under attack is an randomly generated quest.

If you wait too long its an danger the quest will time out and its an chance you will loose settlers.

Enemies attacking then you fast travel is an random encounter.

As I understand settlements with lots of food and water but with low defense has higher chance of getting attacked.

Mind you I have only had 3 attack quests so far. and it was settlements with just one turret.