As mentioned in the Lamp Settings, in XPlane2Blender 3.4, a certain type of "WYSIWYG" light was added:
If the Lamp is a "Spot" (or other non-"Point") lamp and a "Named" or "Param" type and the light name is included in the lights.txt file:
XPlane2Blender will export the Lamp's rotation with What-You-See-Is-What-You-Get behavior
The exporter uses Blender's rotation, even if a param list includes X, Y, Z or DX, DY, DZ components. (The param list must still be valid.)
This means "Custom" lights, unknown light names, and point lights cannot simply use their Blender rotation value to aim the light and an advanced hack must be used instead. While previous versions of this trick have been developed, new optimizations have broken some methods. It is very unlikely that this trick applies to your project. "Custom" lights are a discouraged (but not deprecated) feature; lights.txt will always be updated and should never be edited manually; and, since billboard lights rarely need aiming at all, it is far simpler to use a param light to change the directionality of a billboard than using this trick.
The "Useless Keyframe" Trick
After creating your Lamp, make an animation on it with the following details:
Dataref: "none" (or another non-existent dataref)
Rotation Keyframe 1: Whatever angle you want to aim the light at, a value of 0 for the dataref
Rotation Keyframe 2: A different angle than Keyframe 1's angle, a different value than Keyframe 1's value
How It Works
X-Plane selects which frame of an animation to interpolate to based on the value of the dataref associated with it. Since "none" is not a real dataref, its value will always be 0
Therefore, the animation will always be stuck on Keyframe 1, light pointing in the direction you want it to
Keyframe 2 exists and has "different rotation value/dataref value" pattern because:
An animation must have more than one keyframe or the exporter will not include it in the OBJ
An animation where all keyframes have the same rotation will make the exporter not include it in the OBJ
All dataref values in an animation must be different or X-Plane considers it invalid with potentially unintended results
The "Useless Keyframe" Trick represents the simplest and safest way to meet the above requirements.
If you have a several of these special lights that are aiming in the same direction, you can save time and group them together. Parent all of them to an Empty or Bone, and apply the "Useless Keyframe" trick to only the parent.