Before the recent update when a tz ended all the monsters would turn into regular monsters instantly, now at least their name stays marked as terrorized even after the tz changes. Not sure for how long but noticed this. On d2r not sure about rotw.

Correct. Once spawned the monster remains terrorized

This is new right?

Idk, I haven't played since D2R Launch

Not new, I've noticed this a long time ago

I believe it's once they're loaded, they will be terrorized. However, if you go out of range, they will un-load and then will be unterrorized once re-loaded.

I tested and went into town and came back stayed terrorized this was cows specifically. Before even ones infront of you would instantly go back to regular monsters