last working version: 18.104.22.168 (I skipped .10 and .11)
When a person joins a group or zones in, the bar color defaults to white. Reload doesn't fix it but I think getting into combat does. During combat this is not a problem. Notice from the image that some class colors are fine and the 2 white bars that aren't pointed out are correctly showing priests.
I've seen this in raid and 5-man on my priest, druid, paladin, hunter (the only ones I've tried. I assume it will happen on all classes)