I also use Sandvik full profile inserts almost exclusively.
Does the insert pass a visual check after cutting a bad thread? Only time I've seen this is when the point is gone and the operator doesn't notice.
A faulty profile on the insert is not out of the question, although I have never experienced such on a Sandvik insert. I HAVE on a few others (Vardex for one)
Assuming that the insert is not faulty, and assuming that the gauge is not faulty, the only possibilities are the root or the threadform, as the full profile insert rules out an oversize major. Pitch error should allow the gauge to screw on so far and then stop.
Maybe you are aware but the Sandvik threading system is different to everyone else in that there are FOUR inserts per pitch, not two. External left and right, and internal left and right. Most other systems just use two inserts - External right/internal left, and external left, internal right. Are you absolutely certain you're using the correct one?
There is one other possibility - your lathe may not be syncing the passes perfectly axially. The result is a wider vee - major and minor dia's remain correct, but the wires will sit lower in the thread and pitch diameter will measure undersize.