I couldn’t find anything on here so let me share my easy experience.
Taxis don’t allow Grab into the airport pickup area. In order to take a grab, you need to go to the main road which is like 2-3 min walk.
The main road is just past the car park/motorcycle parking area that is literally across the arrivals exit. You literally walk past the car park and then there’s the main road.
From here, on the main road, a grab can come get you with no problems.