I have added the white balance FX at track level and enabled equalise histogram. When fading the first event in and the last event out there is colour disturbance which makes the fade virtually unusable. Happens with both V17 & V18. Funnily if you were to use the white balance FX on the clip & not track the fade works fine. is this a bug?
It's not a bug. If you use an FX as Track FX it then gets feeded with the frames rendered after Event FX or Event Fades. So your White Balance Track FX (especially with the equalize option enables) tries to compensate your fade. You can't combine FX in such a way (you certainly can but you shouldn't).
You mean if used as Track FX? There are several ways. One is to use/move first and last event to a different track. Another one is to use keyframing for the fx parameters and set them to default for the time of the first and the last video event. I'd prefer the first way.