Hi there-
I have some lens flares on some gameobjects in my scene which ‘pop’/‘disappear’ after they go a certain distance away from the camera, then reappear when closer to the camera. Does anyone know how to adjust the distance at which this happens / eliminate this issue?
(P.S. the lens flar source is well-within the distance of the far clipping plane, and always further than the near clipping plane.)
If its not a Clip Plane issue, then there may be a collider in between camera and flare or may be trigger. If you want to show through specific colliders then you can use LensFlare component which allows you to ignore layers and few more stuff.
Hope this will help you 
If not then explain a little more and post some screenshots…
im not sure but , increase the clipping plane (far) on camera ?
It should be the clipping pane (far) try turning it up to something like 80000.
You should add Lens flare as component to light object and check directional checkbox.