the practice you mentioned, commonly known as "dipping lights" or "flashing headlights," is a common courtesy among truck drivers to communicate with each other on the road.
The act of dipping lights is often used to indicate that it is safe for a truck to merge back into the lane after overtaking another vehicle. It's a way of acknowledging the passing driver's courtesy and providing reassurance that the maneuver was successful.
It's possible that you experienced a specific situation where the truck drivers you encountered didn't use this practice for various reasons. It could be due to changes in driving culture, individual preferences, or other factors.
Keep in mind that customs and practices on the road can vary from region to region and among different groups of drivers. While the tradition of dipping lights is a longstanding one, not all drivers may adhere to it consistently.
If you continue to pass trucks on the road and want to be courteous, you can still use the practice of dipping your lights to indicate that they can merge back into the lane after overtaking your vehicle. Even if some drivers don't reciprocate, it's still a polite gesture that demonstrates consideration for other road users.