If your robot is 90 degrees to the line and all the sensors see the line, then the robot is confused since it's a different condition from a regular line following where only one sensor ever sees the line. You have to code what you want it to do in that case (like maybe stop and turn on the spot until only one sensor sees the line, then the regular code will be valid again and can reacquire the line).
It still might lose the line though if it is an outer sensor that detects the line and the robot starts moving in rhe wrong direction after it turns on the spot. But nothing is perfect. THe only way to reduce this particular problem really is to use more sensors, and even then you can still lose the line, just the robot has to be farther away from the line for it to happen after turning on the spot. (THat's why using more sensors in something like a circular arrangement might help a lot. You have multiple sensors on the front and back to see the line when you are on top of it, and multiple sensors to catch the line when you start to lose it including corners that are 90 degrees and sharper). But it makes things more complicated.