I can create the rainbow flare every time using a down light at home or the sun, but with the lens hood it NEVER occurs. If the lens was tested with the hood, they would not have ever seen it. It is the first lens out of dozens I have owned that really depends so much on use of the hood if a pinpoint bright light source is almost in frame. I dont mind a bit of extra flare the average lens might have without a hood, but the rainbow you can get with this lens is quite dramatic. Having said all that, I cannot recommend this lens highly enough... it is the pick of the Sigma 1.4 trio.Normal flare is not really a problem.
The rainbow flare is an unusual behaviour, they might not have picked it up under typical testing conditions.